Internaional Archives of
Occpatl l
Int Arch Occup Environ Health 44, 1-11 ( 1979)
al'llll
© Springer-Verlag 1979
Original Works
Circadian Rhythms and Behavior of Permanent Nightworkers M Lortie', J Foret2 , C Teiger', and A Laville' 'Laboratoire de Physiologie du Travail-Ergonomie du CNAM, 41, rue Gay-Lussac, F-75005 Paris, France 2Laboratoire de Physiologie du Travail du CNRS, 41, rue Gay-Lussac, F-75005 Paris, France Summary This paper describes rest/activity rhythms of permanent shiftworkers: rotary printers They reported during one week the hours of their sleep onset and of their meals, and their subjective appreciation of tiredness and mood The average sleep duration ( 7 84 h) can be compared with that of day workers and is fairly longer than the duration of day sleep of shiftworkers when they are on nightshift This long sleep can be accounted for by: (1) the early bedtime (around 05 00 h), (2) an adjustment of biological rhythms to the schedule inversion. Other evidence (naps, meal time) supports the hypothesis of a physiological adjustment It is pointed out that this adjustment is fragile: the printers must have very rigid life habits and schedules (sleep, meals) Each time they change this strategy (especially during week-end) it is subjectively felt as detrimental. Key words: Permanent nightwork
Biological rhythms
Sleep
Adjustment
This paper is an attempt to describe the biological rhythms of a group of permanent nightworkers: rotary printers This description is a part of a broader study of their working conditions (Lortie et al , 1978) Working an abnormal schedule determines to a large extent the way in which everyday life is organized and probable effects on the health of the workers For example, weight increases are frequent in shiftworkers: working an abnormal schedule probably causes some metabolic imbalance This condition is very common among printers On the other hand, the interviewed workers constantly emphasized that it is absolutely necessary (and difficult) to take enough care of their sleep The emphasis workers placed on this problem suggests that the adaptation to work schedules is one of the main problems they face. The results reported here come from informal meetings with groups of printers and from questionnaires filled out by some of them The interviews were made in the plant during non-working hours, the questionnaires were filled out at Offprint requests to: J Foret (address see above)
0304-0131/79/0044/0001/$ 2 20
2
M Lortie et al.
home No biological or psychophysiological measurements were performed. Therefore, the following paper is an attempt to "reconstitute" the daily evolution of biological rhythms as a function of the rhythms imposed by the job and by the social environment. The printing company is located in a medium-sized French town and prints a daily newspaper six nights a week The only night-off is Saturday night-Sunday morning The working hours are on a rule steady ( 22 30-04 30) The actual work duration varies according 'to the production demands (special issues, Saturday issues and those for fairs and the end of the year) and technical mishaps. Exceptional requirements result in an increased working time but never in an earlier starting time. This study concerned 13 experienced rotary printers (the job is held only by males) They operate the machines which print the newspapers and therefore work constantly and only by night The other main characteristics of their job are: a heavy physical load: every night each worker carries approximately 1 5 tons of lead plate which he fastens onto the cylinder of the machine the delicate job of controlling the inking (especially at the beginning of the night) a very noisy environment ( 100-110 dB) which requires ear protection. It is interesting to study the printers because, as far as we know, very few papers have been devoted to permanent shiftworkers (Teleky, 1943 ; Conroy et al , 1970; Patkai et al , 1977) In contrast, a lot of research has been carried out concerning shiftworkers, that is those working periodically at night (Foret, 1978). The studies on shiftwork lead to the general conclusion that, even if some adjustment of the biological rhythms occurs, no large phase displacement or even less, no complete inversion, takes place As a result, every return to regular day schedules is rather easy but every new night work period causes the same severe destruction of the biological rhythms This is why we have attempted a description of the biological rhythms in a group of permanent nightworkers, in spite of the limited and indirect evidence we had to work with.
Methods The level of socio-economic homogeneity of the group if fairly high: the typical printer is a middle-aged man; he is living with his wife and two or three children in a house 15 min (by car) away from the working place The necessity of having a quiet place to live in is probably the most important factor in selecting accommodation. There is great stability in the job Workers must rise through a professional hierarchy, resulting in a late start in the job of rotary printer Actually, the job is carried on between the ages of 30 and 65, the age of retirement. The samples can be characterized by: age i = 39 5 years (range 28-52 years), s d = 7 6 years average experience 15 5 years. Our sample is a bit younger than the average of the printer's population of the company (47 years). A preliminary set of results was obtained through interviews with several groups of printers (16 people) Content analysis was performed We collected data concerning sleep, appetite,
Circadian Rhythms of Permanent Nightworkers
3
subjective alertness and mood during the day and what might be called accommodation strategy with time requirements. In the second step, "sleep and nutrition logs" were filled out for one week by 13 printers. They reported estimated sleep onset time, waking up time and, if occurring, intervening wakefulness in bed Food intake time was also reported and the meals were rated in terms of the amount consumed In addition, the workers filled out questionnaires regarding subjective feelings which may reflect information upon the biological cycles (i e , time of least alertness, difficulty of staying awake, etc ). Results 1 Number of Sleep Phases Figure 1 shows that a large proportion of subjects take a nap in addition to their main sleep period (33 naps versus 86 nights). 1
durationlh
2
4
5
6
_,
l
0
a
I
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number of n ar -r
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-
mean duration of nap
mean duration of main
2
sleep period
4
6
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8
a durationlh) In
lu
.8
'O }
~~~~~~~~~~~~~~~~~~~~~~~
L·
6
b MEAN SLEEP
iTi t
mean total sleep
duration and standard deviation
URATION OVER ONE WEEK ln= 13 subjects)
Fig 1 a Mean daily duration of sleep of each subject The above figures are the number of naps reported during one week Above the baseline, shaded columns stand for duration of nap. Below the baseline, white columns stand for duration of main sleep period b Standard deviation of daily sleep of each subject
k
4
M Lortie et al.
Out of 12 subjects, four systematically take a nap and one never does The time these naps occur is remarkable: all of them take place between 20 00 and 22.00. 2 Duration of Sleep Figure 2 shows the sleep duration (n = 86) The sleep durations (naps included) have a median of around 7 5 h and an average of 7 84 h (s d = 1 11 h) if one does not take into account the Saturday-Sunday nights Taking these nights into account would slightly increase the duration and largely the variance of the results Figure 2 shows that the reported naps are long (between 1 and 3 h). 15 NAPS
10 n= 33 I i
l
i
i&
i
i
median
20 -
i
SLEEP
15
working days
n:
10
5
oL
lO I
I
I
I
I
I
saturday night i , 0 C O~~~~~~~~~~~~~~~~~~~~~~~~~~~~ :t P sleep fl I 1 2 3 4 5 6 7
I
I
I
duration
JL_
8
9
10
11 h
Fig 2 Histograms of sleep duration (n = 86) and of nap duration (n = 33) of the 13 subjects (top)
Table 1 Mean and standard deviation of bedtime and of sleep duration as a function of week day (in hours) "Sunday night" means the night between Sunday and Monday Actually, sleep occurs Monday morning Data concerning Saturday night were not reported because of their huge variability Sunday night Monday morning
Monday night Tuesday morning
Tuesday
Wednes day
Thursday
Friday
ff = a=
4 96 -0
5 02 0 23
5 73 0 86
5 48 0 98
5 08 0 73
5 53 0 80
Sleep duration ffm= a=
8 42 0 64
8 40 0 98
7 67 1 06
7 52 1 55
8 35 0 80
7 02 1 18
Bedtime
Circadian Rhythms of Permanent Nightworkers
5
The range of average sleep duration (naps included) in individuals is small (7.43 to 9 h) as Figure 1 shows There is no significant difference in total daily sleep duration between "nappers" and "non-nappers" Perhaps this non-significance is due to the small number of subjects. Table 1 shows sleep duration (naps included) and bedtime as a function of the day of the week. One can see that: at the beginning of a new week (Sunday-Monday night) the workers go to bed as early as possible and they sleep longer, the dispersion of the results is extremely small That might reflect a recovery sleep due to the sleep deficit which results from the weekend for most of the workers; sleep duration is shorter when bedtime is later In the case of the printers, bedtime is a close function of the overtime due to the printing of special issues. During the week of study, this work was carried out on Wednesday night and on Friday night Perhaps the non-significant trend towards sleep getting shorter between Sunday and Friday is due to this overtime At least, it is clear that there is no lengthening of sleep time through the days of the week. 3 Behavior and Opinion About Sleep Sleeping pills: five printers (out of 12) take sleeping pills, three of them regularly. One of the explanations mentioned by all of them is the need of sleeping during "normal" hours, Saturday night, and of being synchronized with their surrounding group. only three of them report difficulties in going to sleep, but six (that is to say half of the sample) feel tired while awakening around 13 00 However, most of them (eight) wake up spontaneously as a rule; the prevailing opinion is that they have a light or very light sleep (eight out of 13); during the interviews, troubles in organizing waking time and sleeping time as a function of working time are often mentioned. The printers emphasize how rigid their daily schedules have to be: evening dinner (the main meal) and nap, may not and cannot in any case be taken at different times than usual Similarly, they think that going to bed immediately after finishing work is essential: delays in "bonder" the newspaper or the overtime due to specially heavy pagination are strongly disliked: the fear of lacking sleep is constantly repeated, even though the daily average durations are not far from the durations usually found in daytime workers. Summing up, sleep appears to be a serious matter and a problem they constantly have to solve. 4 Wakefulness If asked how they feel at different times of the day, only three printers answer that they have troubles in staying awake in the night But, all of them report a "difficult moment", especially around 03 00 after the break It is interesting to note that, after 01 00, the process of printing requires much less intervening and therefore less vigilance (except in case of technical incidents, like paper breaks).
6
M Lortie et al.
5 Nutrition The distribution in time of meals is original: the printers on average take only one full meal a day: the dinner The rest of the daily food consists essentially of snacks and nibbling The midday lunch is usually taken without appetite and must be regarded as a breakfast; during the night, all printers take only snacks. It is clear that the need for food, reflected in appetite, shows circadian evolution which is shifted in time with respect to that of day workers. In all the previously presented results, the weekend appears to be a severe disturbance, during which time the printers try to put in agreement their activities, their sleep and their meals with the surrounding community They unanimously attest that they do not succeed and that, in contrast, they experience the worst time of the week every Sunday night, when they have to start the work again In the same way, when they start a period of holidays, getting readapted to the diurnal life takes from a few days to one week. When asked about the worst time of year, the majority answers months prior to holidays (May, June, or July), contrary to what could be expected Storage of fatigue seems to play a larger part in this opinion than the difficulties of readaptation to night schedules (mentioned only once) Other answers are December (mentioned twice) because of the usual extra work at the end of the year.
Discussion It is difficult to draw conclusions from the high frequency of naps which was reported (33 naps out of 86 nights) On one hand, the literature (Webb, 1971) contains very little data concerning naps On the other hand, the fact that naps exist or do not exist is due to a choice of society rather than to biological needs. The trend now-at least in Northern Europe-favors concentrated daily working schedules, precluding naps That choice does not give any indications as to what would happen if human beings had completely free schedules. Nevertheless, we think that the present results raise the problem of monophasicity of human sleep: is it certain that the entire sleep must occur in one period? The case of the printers seems to be an extreme one Although the actual depth of sleep during these naps is not known, their length (between 1 and 3 h) indicates that it is unlikely that they consist only of light sleep According to Evans et al 's (1977) rationale, it may be hypothesized that the naps of printers-at least in systematic "nappers"-are a non-negligible part of their daily amount of sleep. This biphasicity of sleep could be an original feature of the compromise reached by some permanent nightworkers in order to live up to the demands of their very special schedules It would explain why the evening nap is so obligatory, indeed "sacred" and why it must be respected by the environment -that is to say essentially by the family-which obviously raises some difficult problems. On the other hand, this hypothesis is supported by the lack of difference in the length of daily sleep among "nappers" and "non-nappers" This result is not in agreement with several other studies (e g , Tune, 1969) which suggest that naps
7
Circadian Rhythms of Permanent Nightworkers main catecholamine I
RESS RINTERS
t press min Mr Ot Sl arkt un lits
lunch
Sinner SL_ SLr
20h
2
REE 8
ln Plilil
SEERPt
12h
R
WORK ,
SCEDOULES I6h
20 h
Fig 3 Circadian evolution of general arousal (1) of a press printer (above); (2) of a regular day worker (below) The ordinate scale is arbitrary The dotted line represents the main sleep period of a shiftworker when he is on nightshift
extend the daily sleep amount and that they might reflect a higher sleep need in nappers. The data concerning sleep length can be summarized as follows: 1 On the average, there is no serious quantitative sleep deficit In fact, during the week, only the two first nights (Sunday and Monday nights) can be regarded as recovery nights after the disturbance due to the weekend Variance analysis shows a high level of homogeneity between subjects: most of the variance of sleep duration is actually caused by variations as a function of week day. 2 The printers maintain practically normal sleep amounts in contrast with most of the shiftworkers Workers committed to rotating schedules reported without exception shortened sleep duration during night work (Foret, 1978 ; Mott et al , 1965 ; Rutenfranz et al , 1977). Even though it is still impossible to explain why sleep is so necessary from a biological standpoint-one may agree with Knauth and Rutenfranz ( 1972) that analysis of sleep can be used as an objective criterion in the assessment of shift rotas Of course the duration, even if it is not sufficient to estimate sleep quality, is an important index The normal sleep duration implies that printers go to bed at a time which, biologically speaking, allows this sleep duration Additional evidence is that any delay of bedtime results in a decreased sleep length, in good agreement with all the studies of daytime sleep (Foret, 1973 ; Nachreiner et al , 1975). Despite lack of precise biological measurements, we tried to estimate the circadian evolution of general arousal It is a physiological concept not precisely defined but whose biological and psychophysiological correlations are clear cut: its peak is in high correlation with the maximum of body temperature, heart rate, adrenaline excretion rate, etc It is also the time of day that responses to different vigilance tests are best Its minimum coincides with the activity minimum, that is practically in the middle of the sleep period In people with normal living schedules, a curve with a maximum (generally in the afternoon) and a minimum (at the end of the night) can be plotted every 24 h.
8
M Lortie et al.
In Fig 3, general arousal of a printer is assessed (upper line) in comparison with an individual with regular working hours (lower line) These results come from many studies (e g , Colquhoun et al , 1968, 1969) which deal with responses of human subjects to abnormal schedules Because we did not take measurements directly related to the general level, such as body temperature, heart rate, 17 OHCS, etc , we collected the data coming from questionnaires and a few results according to Patkai et al (1977) We plotted the evolution of general arousal as to correlate with as many data as possible. Between a "normal" worker and a printer, phase differences are as follows: beginning of working time: around 14-15 h (respectively 08 00 h and 22 30 h) beginning of sleep time: around 6 h (respectively 23 00 h and 06 00 h). Of primary importance is that between the two cases general arousal and sleep have their peaks shifted by the same amount: about 6 h. maximum: between 16 00 h and 22 00 h minimum: between 05 00 h and 12 00 h. Therefore, sleep periods in both cases, in the day worker as well as the printer, are located around the minimum of general arousal, that is to say at a propitious time In contrast, sleep duration of the occasional night worker is plotted (dotted line) on the lower curve: his sleep is short and of poor quality because the circadian evolution of his general arousal does not differ grossly from that of a day worker Hence, the sleep period is located on an ascending part of the curve, that is to say an unfavorable one. The printer's working period does not have the same cyclical location as the day worker's: at least part of his shift occurs in a phase during which the organism is "getting to sleep" That is reflected in the troubles the printers reported to experience during the last two hours of duty between 03 00 h and 05.00 h and the easy way in which they fall asleep immediately after work. This result is in agreement with many previous studies (e g , Bjerner et al., 1955 ; Ulich, 1964) which reported the lowest point of performance level at about 03 00 h in various cases of working schedules. It would be interesting to get detailed results concerning food intake The data collected during the interviews about nutrition habits are consistent with the estimated curve of the arousal The midday lunch is light because the subjects are not hungry One would infer that it is located too early in the ascending part of the curve In contrast, the evening dinner takes place at 20 00 h, not far from the activation peak, which makes it resembles the lunch of a day worker It has been often pointed out that shiftwork alters the distribution of food intake over the 24-h period (Debry and Bleyer, 1972) and has disturbing effects on appetite and digestion (Ulich, 1958 ; Wyatt and Marriott, 1953) It is well known now that this kind of trouble is due to the antagonism between the circadian patterns of feeding behavior and the environmental demands It seems that printers are able to keep an efficient compromise between the daily evolution of appetite (reflected by meal time and food amount) and social habits, particularly the family life. For lack of data, inter-individual variations have not been taken into account (e.g , differences between naturally long and short sleepers, between morning people and evening people) These differences are likely to be important in ex-
Circadian Rhythms of Permanent Nightworkers
9
plaining why some printers have a systematic evening nap while the average curve cannot explain it. The preceding results lead to the conclusion that permanent nightwork result in an adjustment of time evolution of activation and, more generally, of biological rhythms Additional evidence is that cleaners who do not usually work at night but occasionally replace the printers, take a nap not when they work at night but around 14 00 h, as do day workers They probably do not have the adjustment which seems to occur only in permanent nightwork. Another piece of evidence of this adjustment is the evolution of sleep length during the week In workers on night shift (Foret and Benoit, 1978) and in printers with an alternating schedule (one week day, one week night), (Patkai et al., 1977), morning sleep varies as follows: immediately after inversion, day sleep is short, probably because of the extreme disturbance of biological rhythms. During the following days, it tends to get longer as an effect of the sleep deficit due to inversion and probably because an adjustment to inversion progressively takes place. In printers, however, the daily sleep amount tends to decrease as a function of the day of the week This leads to the conclusion that an adjustment occurs as early as Monday and that it is efficient enough to permit a regular sleep length. Nevertheless, the adjustment of biological rhythms in printers is obviously fragile The following evidence supports this assumption: the relationship between sleep length and bedtime As Table I shows, if bedtime is delayed, sleep length is shortened approximatively by the same amount, although this results in sleep deficit This agrees with the results of Foret and Benoit ( 1974), obtained from shiftworkers. the conditions the printers have to comply with if they want to succeed; type of accomodation (houses as quiet as possible), lifestyle the family must accept to allow them to sleep at precise and convenient times; the insistent way in which they describe the temporal organization of their life as a major and permanent concern, they report a light or very light morning sleep, they wake up easily but in poor shape, the weekend during which their work schedules are not complied with results in increased fatigue Behavior on Mondays reflects a need for recovery On Saturday night, the printers try to fit their schedule with the environment Many of them take sleeping pills to get to sleep early enough Obviously, few succeed and, Sunday morning, get up not having slept enough. That is why the night between Sunday and Monday (the first night of work) is rated the most difficult of the week, probably because of accumulated fatigue. Monday night is then a recovery night. As a general conclusion, permanent night workers such as printers reveal to some extent an adjustment of their biological rhythms In other shiftworkers committed to a rotating schedule, this adjustment can always be questioned This fact shows an essential difference between them and the printers Nevertheless, even though printers seem not to be "chronically sleep deprived", in contrast to some shiftworkers, many of the answers suggest that their adjustment is fragile.
10
M Lortie et al.
The printers have found a compromise between three kinds of temporal demands: their "internal clock" (biological rhythms), their working schedule and their social life (family, friends, surrounding society) However, the collected data imply that the degree of freedom allowed by this "modus vivendi" is very limited in the way they can organize their daily life. If this adjustment exists, it should be reflected by a high level of satisfaction. In fact, like in Wyatt and Marriott ( 1953), we studied people who were longservice nightworkers Besides, the severe demands as regards the timing in the press industry were never questioned by the men included in this inquiry That is why we did not put the emphasis on the problem of satisfaction Many studies (like Wyatt and Marriott, 1953 ; Akerstedt and Torsvall, 1978) showed that, when a choice is allowed or when economic pressures decrease the extent of shiftwork in a company, a large majority of men prefer day work to night work In the present study, we presume that the results concerning satisfaction would be completely changed if the workers were given the choice between various working schedules, for example if the press company printed a morning and an evening edition In addition one must ask the question of the long-term cost of this adaptation The poor quality of social life is obvious The question becomes: how to live successfully with others when your rest schedules are compelling and out of phase with the environment? There might be a long term biological cost which results in a higher rate of morbidity of these printers when compared with others working in printing companies (Lortie et al , 1978) and perhaps in overmortality as well This latter hypothesis is the topic of a study in progress Of course, night work itself is not the only factor The point is rather to examine the relationship between night schedules and other negative components of the working situation such as noise, toxic exposure and work load.
References Akerstedt, T , Torsvall, L : Experimental changes in shift schedules Their effects on well-being. Ergonomics 21, 849-856 (1978)
Bjerner, B , Holm, A , Swensson, A : Diurnal variations in mental performance: a study of threeshift workers Br J Ind Med 12, 103-110 (1955) Colquhoun, W P , Blake, M J F , Edwards, R S : Experimental studies of shift work I A comparison of "rotating" and "stabilized" four-hour shift systems Ergonomics 11,437-453 (1968 a) Colquhoun, W P , Blake, M J F , Edwards, R S : Experimental studies of shift work II.
Stabilized eight-hour shift systems Ergonomics 11, 527-546 (1968 b) Colquhoun, W P , Blake, M J F , Edwards, R S : Experimental studies of shift work III. Stabilized twelve-hour shift systems Ergonomics 12, 865-882 (1969) Conroy, R T W M , Elliott, A L , Mills, J N : Circadian rhythms in plasma concentration of 11-Hydroxycorticosteroids in men working in night shift and in permanent night workers. Br J Med Psychol 27, 170-174 (1970) Debry, G , Bleyer, R : Influence du rythme des 3 x 8 sur l'alimentation des travailleurs In:
Alimentation et travail, G Debry, R Bleyer (eds ), pp 153-172 Paris: Masson 1972 Evans, F J , Cook, M R , Cohen, H D , Orne, E C , Orne, M T : Appetitive and replacement naps: EEG and behavior Science 197, 687-689 (1977) Foret, J : Sommeil et horaires de travail irr6guliers Dissertation, Lille 1973 Foret, J : L'homme en activity et son sommeil Le Travail Humain 41, 43-54 (1978)
Circadian Rhythms of Permanent Nightworkers
11
Foret, J , Benoit, O : Structure du sommeil chez des travailleurs a horaires alternants Electroencephalogr Clin Neurophysiol 37, 337-344 (1974) Foret, J , Benoit, O : Etude du sommeil de travailleurs horaires alternants Adaptation et recuperation dans le cas de rotation rapide de poste ( 3-4 jours) Eur J Appl Physiol 38, 71-82 (1978) Knauth, P , Rutenfranz, J : Untersuchungen zum Problem des Schlafverhaltens bei experimenteller Schichtarbeit Int Arch Arbeitsmed 30, 1-22 (1972) Lortie, M , Teiger, C , Laville, A : Conditions de travail, 6tat de sante et mortality chez des travailleurs de nuit permanents (rotativistes de presse) Laboratoire de Physiologie du Travail, CNAM (in press) Mott, P E , Mann, F C , McLoughlin, Q , Warwick, D P : Shiftwork Ann Arbor: University of Michigan Press 1965 Nachreiner, R , Frielingsdorf, R , Romahn, R , Knauth, P , Kuhlmann, W , Klimmer, R , Rutenfranz, J , Werner, E : Schichtarbeit bei kontinuierlicher Produktion Bundesanstalt fir Arbeitsschutz und Unfallforschung Dortmund, 1975 Patkai, P , Akerstedt, T , Pettersson, K : Field studies of shiftwork I Temporal patterns in psychophysiological activation in permanent night workers Ergonomics 20, 611-619 (1977) Reinberg, A , Migraine, C , Apfelbaum, M , Brigant, L , Ghata, J , Vieux, N , Laporte, A : Circadian and ultradian rhythms in the feeding behavior and nutrient intake of oil refinery operators on a rapidly rotating shift system Ergonomics 21, 862 (1978) Rutenfranz, J , Colquhoun, W P , Knauth, P , Ghata, J : Biomedical and psychosocial aspects of shift work Scand J Work Environ Health 3, 165-182 (1977) Teleky, L : Problems of night work Influences on health and efficiency Ind Med Surg 12, 758-779 (1943) Tune, G S : Sleep and wakefulness in a group of shiftworkers Br J Ind Med 26, 54-58 (1969) Ulich, E : Zur Frage der Belastung des arbeitenden Menschen durch Nacht und Schichtarbeit. Psychol Rundschau 8, 42-61 (1957) Ulich, E : Schicht und Nachtarbeit im Betrieb K 61n, Opladen: Westdeutscher Verlag 1964 Webb, W B : Sleep behavior as a biorhythm In: Biological rhythms and human performance, W P Colquhoun (ed ), pp 149-178 London: Academic Press 1971 Wyatt, S , Marriott, R : Night work and shift changes Br J Ind Med 10, 164-172 (1953) Received November 21, 1978 / Accepted March 8, 1979
Occpatl l
Int Arch Occup Environ Health 44, 1-11 ( 1979)
al'llll
© Springer-Verlag 1979
Original Works
Circadian Rhythms and Behavior of Permanent Nightworkers M Lortie', J Foret2 , C Teiger', and A Laville' 'Laboratoire de Physiologie du Travail-Ergonomie du CNAM, 41, rue Gay-Lussac, F-75005 Paris, France 2Laboratoire de Physiologie du Travail du CNRS, 41, rue Gay-Lussac, F-75005 Paris, France Summary This paper describes rest/activity rhythms of permanent shiftworkers: rotary printers They reported during one week the hours of their sleep onset and of their meals, and their subjective appreciation of tiredness and mood The average sleep duration ( 7 84 h) can be compared with that of day workers and is fairly longer than the duration of day sleep of shiftworkers when they are on nightshift This long sleep can be accounted for by: (1) the early bedtime (around 05 00 h), (2) an adjustment of biological rhythms to the schedule inversion. Other evidence (naps, meal time) supports the hypothesis of a physiological adjustment It is pointed out that this adjustment is fragile: the printers must have very rigid life habits and schedules (sleep, meals) Each time they change this strategy (especially during week-end) it is subjectively felt as detrimental. Key words: Permanent nightwork
Biological rhythms
Sleep
Adjustment
This paper is an attempt to describe the biological rhythms of a group of permanent nightworkers: rotary printers This description is a part of a broader study of their working conditions (Lortie et al , 1978) Working an abnormal schedule determines to a large extent the way in which everyday life is organized and probable effects on the health of the workers For example, weight increases are frequent in shiftworkers: working an abnormal schedule probably causes some metabolic imbalance This condition is very common among printers On the other hand, the interviewed workers constantly emphasized that it is absolutely necessary (and difficult) to take enough care of their sleep The emphasis workers placed on this problem suggests that the adaptation to work schedules is one of the main problems they face. The results reported here come from informal meetings with groups of printers and from questionnaires filled out by some of them The interviews were made in the plant during non-working hours, the questionnaires were filled out at Offprint requests to: J Foret (address see above)
0304-0131/79/0044/0001/$ 2 20
2
M Lortie et al.
home No biological or psychophysiological measurements were performed. Therefore, the following paper is an attempt to "reconstitute" the daily evolution of biological rhythms as a function of the rhythms imposed by the job and by the social environment. The printing company is located in a medium-sized French town and prints a daily newspaper six nights a week The only night-off is Saturday night-Sunday morning The working hours are on a rule steady ( 22 30-04 30) The actual work duration varies according 'to the production demands (special issues, Saturday issues and those for fairs and the end of the year) and technical mishaps. Exceptional requirements result in an increased working time but never in an earlier starting time. This study concerned 13 experienced rotary printers (the job is held only by males) They operate the machines which print the newspapers and therefore work constantly and only by night The other main characteristics of their job are: a heavy physical load: every night each worker carries approximately 1 5 tons of lead plate which he fastens onto the cylinder of the machine the delicate job of controlling the inking (especially at the beginning of the night) a very noisy environment ( 100-110 dB) which requires ear protection. It is interesting to study the printers because, as far as we know, very few papers have been devoted to permanent shiftworkers (Teleky, 1943 ; Conroy et al , 1970; Patkai et al , 1977) In contrast, a lot of research has been carried out concerning shiftworkers, that is those working periodically at night (Foret, 1978). The studies on shiftwork lead to the general conclusion that, even if some adjustment of the biological rhythms occurs, no large phase displacement or even less, no complete inversion, takes place As a result, every return to regular day schedules is rather easy but every new night work period causes the same severe destruction of the biological rhythms This is why we have attempted a description of the biological rhythms in a group of permanent nightworkers, in spite of the limited and indirect evidence we had to work with.
Methods The level of socio-economic homogeneity of the group if fairly high: the typical printer is a middle-aged man; he is living with his wife and two or three children in a house 15 min (by car) away from the working place The necessity of having a quiet place to live in is probably the most important factor in selecting accommodation. There is great stability in the job Workers must rise through a professional hierarchy, resulting in a late start in the job of rotary printer Actually, the job is carried on between the ages of 30 and 65, the age of retirement. The samples can be characterized by: age i = 39 5 years (range 28-52 years), s d = 7 6 years average experience 15 5 years. Our sample is a bit younger than the average of the printer's population of the company (47 years). A preliminary set of results was obtained through interviews with several groups of printers (16 people) Content analysis was performed We collected data concerning sleep, appetite,
Circadian Rhythms of Permanent Nightworkers
3
subjective alertness and mood during the day and what might be called accommodation strategy with time requirements. In the second step, "sleep and nutrition logs" were filled out for one week by 13 printers. They reported estimated sleep onset time, waking up time and, if occurring, intervening wakefulness in bed Food intake time was also reported and the meals were rated in terms of the amount consumed In addition, the workers filled out questionnaires regarding subjective feelings which may reflect information upon the biological cycles (i e , time of least alertness, difficulty of staying awake, etc ). Results 1 Number of Sleep Phases Figure 1 shows that a large proportion of subjects take a nap in addition to their main sleep period (33 naps versus 86 nights). 1
durationlh
2
4
5
6
_,
l
0
a
I
I
number of n ar -r
r
-
mean duration of nap
mean duration of main
2
sleep period
4
6
r
_
8
a durationlh) In
lu
.8
'O }
~~~~~~~~~~~~~~~~~~~~~~~
L·
6
b MEAN SLEEP
iTi t
mean total sleep
duration and standard deviation
URATION OVER ONE WEEK ln= 13 subjects)
Fig 1 a Mean daily duration of sleep of each subject The above figures are the number of naps reported during one week Above the baseline, shaded columns stand for duration of nap. Below the baseline, white columns stand for duration of main sleep period b Standard deviation of daily sleep of each subject
k
4
M Lortie et al.
Out of 12 subjects, four systematically take a nap and one never does The time these naps occur is remarkable: all of them take place between 20 00 and 22.00. 2 Duration of Sleep Figure 2 shows the sleep duration (n = 86) The sleep durations (naps included) have a median of around 7 5 h and an average of 7 84 h (s d = 1 11 h) if one does not take into account the Saturday-Sunday nights Taking these nights into account would slightly increase the duration and largely the variance of the results Figure 2 shows that the reported naps are long (between 1 and 3 h). 15 NAPS
10 n= 33 I i
l
i
i&
i
i
median
20 -
i
SLEEP
15
working days
n:
10
5
oL
lO I
I
I
I
I
I
saturday night i , 0 C O~~~~~~~~~~~~~~~~~~~~~~~~~~~~ :t P sleep fl I 1 2 3 4 5 6 7
I
I
I
duration
JL_
8
9
10
11 h
Fig 2 Histograms of sleep duration (n = 86) and of nap duration (n = 33) of the 13 subjects (top)
Table 1 Mean and standard deviation of bedtime and of sleep duration as a function of week day (in hours) "Sunday night" means the night between Sunday and Monday Actually, sleep occurs Monday morning Data concerning Saturday night were not reported because of their huge variability Sunday night Monday morning
Monday night Tuesday morning
Tuesday
Wednes day
Thursday
Friday
ff = a=
4 96 -0
5 02 0 23
5 73 0 86
5 48 0 98
5 08 0 73
5 53 0 80
Sleep duration ffm= a=
8 42 0 64
8 40 0 98
7 67 1 06
7 52 1 55
8 35 0 80
7 02 1 18
Bedtime
Circadian Rhythms of Permanent Nightworkers
5
The range of average sleep duration (naps included) in individuals is small (7.43 to 9 h) as Figure 1 shows There is no significant difference in total daily sleep duration between "nappers" and "non-nappers" Perhaps this non-significance is due to the small number of subjects. Table 1 shows sleep duration (naps included) and bedtime as a function of the day of the week. One can see that: at the beginning of a new week (Sunday-Monday night) the workers go to bed as early as possible and they sleep longer, the dispersion of the results is extremely small That might reflect a recovery sleep due to the sleep deficit which results from the weekend for most of the workers; sleep duration is shorter when bedtime is later In the case of the printers, bedtime is a close function of the overtime due to the printing of special issues. During the week of study, this work was carried out on Wednesday night and on Friday night Perhaps the non-significant trend towards sleep getting shorter between Sunday and Friday is due to this overtime At least, it is clear that there is no lengthening of sleep time through the days of the week. 3 Behavior and Opinion About Sleep Sleeping pills: five printers (out of 12) take sleeping pills, three of them regularly. One of the explanations mentioned by all of them is the need of sleeping during "normal" hours, Saturday night, and of being synchronized with their surrounding group. only three of them report difficulties in going to sleep, but six (that is to say half of the sample) feel tired while awakening around 13 00 However, most of them (eight) wake up spontaneously as a rule; the prevailing opinion is that they have a light or very light sleep (eight out of 13); during the interviews, troubles in organizing waking time and sleeping time as a function of working time are often mentioned. The printers emphasize how rigid their daily schedules have to be: evening dinner (the main meal) and nap, may not and cannot in any case be taken at different times than usual Similarly, they think that going to bed immediately after finishing work is essential: delays in "bonder" the newspaper or the overtime due to specially heavy pagination are strongly disliked: the fear of lacking sleep is constantly repeated, even though the daily average durations are not far from the durations usually found in daytime workers. Summing up, sleep appears to be a serious matter and a problem they constantly have to solve. 4 Wakefulness If asked how they feel at different times of the day, only three printers answer that they have troubles in staying awake in the night But, all of them report a "difficult moment", especially around 03 00 after the break It is interesting to note that, after 01 00, the process of printing requires much less intervening and therefore less vigilance (except in case of technical incidents, like paper breaks).
6
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5 Nutrition The distribution in time of meals is original: the printers on average take only one full meal a day: the dinner The rest of the daily food consists essentially of snacks and nibbling The midday lunch is usually taken without appetite and must be regarded as a breakfast; during the night, all printers take only snacks. It is clear that the need for food, reflected in appetite, shows circadian evolution which is shifted in time with respect to that of day workers. In all the previously presented results, the weekend appears to be a severe disturbance, during which time the printers try to put in agreement their activities, their sleep and their meals with the surrounding community They unanimously attest that they do not succeed and that, in contrast, they experience the worst time of the week every Sunday night, when they have to start the work again In the same way, when they start a period of holidays, getting readapted to the diurnal life takes from a few days to one week. When asked about the worst time of year, the majority answers months prior to holidays (May, June, or July), contrary to what could be expected Storage of fatigue seems to play a larger part in this opinion than the difficulties of readaptation to night schedules (mentioned only once) Other answers are December (mentioned twice) because of the usual extra work at the end of the year.
Discussion It is difficult to draw conclusions from the high frequency of naps which was reported (33 naps out of 86 nights) On one hand, the literature (Webb, 1971) contains very little data concerning naps On the other hand, the fact that naps exist or do not exist is due to a choice of society rather than to biological needs. The trend now-at least in Northern Europe-favors concentrated daily working schedules, precluding naps That choice does not give any indications as to what would happen if human beings had completely free schedules. Nevertheless, we think that the present results raise the problem of monophasicity of human sleep: is it certain that the entire sleep must occur in one period? The case of the printers seems to be an extreme one Although the actual depth of sleep during these naps is not known, their length (between 1 and 3 h) indicates that it is unlikely that they consist only of light sleep According to Evans et al 's (1977) rationale, it may be hypothesized that the naps of printers-at least in systematic "nappers"-are a non-negligible part of their daily amount of sleep. This biphasicity of sleep could be an original feature of the compromise reached by some permanent nightworkers in order to live up to the demands of their very special schedules It would explain why the evening nap is so obligatory, indeed "sacred" and why it must be respected by the environment -that is to say essentially by the family-which obviously raises some difficult problems. On the other hand, this hypothesis is supported by the lack of difference in the length of daily sleep among "nappers" and "non-nappers" This result is not in agreement with several other studies (e g , Tune, 1969) which suggest that naps
7
Circadian Rhythms of Permanent Nightworkers main catecholamine I
RESS RINTERS
t press min Mr Ot Sl arkt un lits
lunch
Sinner SL_ SLr
20h
2
REE 8
ln Plilil
SEERPt
12h
R
WORK ,
SCEDOULES I6h
20 h
Fig 3 Circadian evolution of general arousal (1) of a press printer (above); (2) of a regular day worker (below) The ordinate scale is arbitrary The dotted line represents the main sleep period of a shiftworker when he is on nightshift
extend the daily sleep amount and that they might reflect a higher sleep need in nappers. The data concerning sleep length can be summarized as follows: 1 On the average, there is no serious quantitative sleep deficit In fact, during the week, only the two first nights (Sunday and Monday nights) can be regarded as recovery nights after the disturbance due to the weekend Variance analysis shows a high level of homogeneity between subjects: most of the variance of sleep duration is actually caused by variations as a function of week day. 2 The printers maintain practically normal sleep amounts in contrast with most of the shiftworkers Workers committed to rotating schedules reported without exception shortened sleep duration during night work (Foret, 1978 ; Mott et al , 1965 ; Rutenfranz et al , 1977). Even though it is still impossible to explain why sleep is so necessary from a biological standpoint-one may agree with Knauth and Rutenfranz ( 1972) that analysis of sleep can be used as an objective criterion in the assessment of shift rotas Of course the duration, even if it is not sufficient to estimate sleep quality, is an important index The normal sleep duration implies that printers go to bed at a time which, biologically speaking, allows this sleep duration Additional evidence is that any delay of bedtime results in a decreased sleep length, in good agreement with all the studies of daytime sleep (Foret, 1973 ; Nachreiner et al , 1975). Despite lack of precise biological measurements, we tried to estimate the circadian evolution of general arousal It is a physiological concept not precisely defined but whose biological and psychophysiological correlations are clear cut: its peak is in high correlation with the maximum of body temperature, heart rate, adrenaline excretion rate, etc It is also the time of day that responses to different vigilance tests are best Its minimum coincides with the activity minimum, that is practically in the middle of the sleep period In people with normal living schedules, a curve with a maximum (generally in the afternoon) and a minimum (at the end of the night) can be plotted every 24 h.
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M Lortie et al.
In Fig 3, general arousal of a printer is assessed (upper line) in comparison with an individual with regular working hours (lower line) These results come from many studies (e g , Colquhoun et al , 1968, 1969) which deal with responses of human subjects to abnormal schedules Because we did not take measurements directly related to the general level, such as body temperature, heart rate, 17 OHCS, etc , we collected the data coming from questionnaires and a few results according to Patkai et al (1977) We plotted the evolution of general arousal as to correlate with as many data as possible. Between a "normal" worker and a printer, phase differences are as follows: beginning of working time: around 14-15 h (respectively 08 00 h and 22 30 h) beginning of sleep time: around 6 h (respectively 23 00 h and 06 00 h). Of primary importance is that between the two cases general arousal and sleep have their peaks shifted by the same amount: about 6 h. maximum: between 16 00 h and 22 00 h minimum: between 05 00 h and 12 00 h. Therefore, sleep periods in both cases, in the day worker as well as the printer, are located around the minimum of general arousal, that is to say at a propitious time In contrast, sleep duration of the occasional night worker is plotted (dotted line) on the lower curve: his sleep is short and of poor quality because the circadian evolution of his general arousal does not differ grossly from that of a day worker Hence, the sleep period is located on an ascending part of the curve, that is to say an unfavorable one. The printer's working period does not have the same cyclical location as the day worker's: at least part of his shift occurs in a phase during which the organism is "getting to sleep" That is reflected in the troubles the printers reported to experience during the last two hours of duty between 03 00 h and 05.00 h and the easy way in which they fall asleep immediately after work. This result is in agreement with many previous studies (e g , Bjerner et al., 1955 ; Ulich, 1964) which reported the lowest point of performance level at about 03 00 h in various cases of working schedules. It would be interesting to get detailed results concerning food intake The data collected during the interviews about nutrition habits are consistent with the estimated curve of the arousal The midday lunch is light because the subjects are not hungry One would infer that it is located too early in the ascending part of the curve In contrast, the evening dinner takes place at 20 00 h, not far from the activation peak, which makes it resembles the lunch of a day worker It has been often pointed out that shiftwork alters the distribution of food intake over the 24-h period (Debry and Bleyer, 1972) and has disturbing effects on appetite and digestion (Ulich, 1958 ; Wyatt and Marriott, 1953) It is well known now that this kind of trouble is due to the antagonism between the circadian patterns of feeding behavior and the environmental demands It seems that printers are able to keep an efficient compromise between the daily evolution of appetite (reflected by meal time and food amount) and social habits, particularly the family life. For lack of data, inter-individual variations have not been taken into account (e.g , differences between naturally long and short sleepers, between morning people and evening people) These differences are likely to be important in ex-
Circadian Rhythms of Permanent Nightworkers
9
plaining why some printers have a systematic evening nap while the average curve cannot explain it. The preceding results lead to the conclusion that permanent nightwork result in an adjustment of time evolution of activation and, more generally, of biological rhythms Additional evidence is that cleaners who do not usually work at night but occasionally replace the printers, take a nap not when they work at night but around 14 00 h, as do day workers They probably do not have the adjustment which seems to occur only in permanent nightwork. Another piece of evidence of this adjustment is the evolution of sleep length during the week In workers on night shift (Foret and Benoit, 1978) and in printers with an alternating schedule (one week day, one week night), (Patkai et al., 1977), morning sleep varies as follows: immediately after inversion, day sleep is short, probably because of the extreme disturbance of biological rhythms. During the following days, it tends to get longer as an effect of the sleep deficit due to inversion and probably because an adjustment to inversion progressively takes place. In printers, however, the daily sleep amount tends to decrease as a function of the day of the week This leads to the conclusion that an adjustment occurs as early as Monday and that it is efficient enough to permit a regular sleep length. Nevertheless, the adjustment of biological rhythms in printers is obviously fragile The following evidence supports this assumption: the relationship between sleep length and bedtime As Table I shows, if bedtime is delayed, sleep length is shortened approximatively by the same amount, although this results in sleep deficit This agrees with the results of Foret and Benoit ( 1974), obtained from shiftworkers. the conditions the printers have to comply with if they want to succeed; type of accomodation (houses as quiet as possible), lifestyle the family must accept to allow them to sleep at precise and convenient times; the insistent way in which they describe the temporal organization of their life as a major and permanent concern, they report a light or very light morning sleep, they wake up easily but in poor shape, the weekend during which their work schedules are not complied with results in increased fatigue Behavior on Mondays reflects a need for recovery On Saturday night, the printers try to fit their schedule with the environment Many of them take sleeping pills to get to sleep early enough Obviously, few succeed and, Sunday morning, get up not having slept enough. That is why the night between Sunday and Monday (the first night of work) is rated the most difficult of the week, probably because of accumulated fatigue. Monday night is then a recovery night. As a general conclusion, permanent night workers such as printers reveal to some extent an adjustment of their biological rhythms In other shiftworkers committed to a rotating schedule, this adjustment can always be questioned This fact shows an essential difference between them and the printers Nevertheless, even though printers seem not to be "chronically sleep deprived", in contrast to some shiftworkers, many of the answers suggest that their adjustment is fragile.
10
M Lortie et al.
The printers have found a compromise between three kinds of temporal demands: their "internal clock" (biological rhythms), their working schedule and their social life (family, friends, surrounding society) However, the collected data imply that the degree of freedom allowed by this "modus vivendi" is very limited in the way they can organize their daily life. If this adjustment exists, it should be reflected by a high level of satisfaction. In fact, like in Wyatt and Marriott ( 1953), we studied people who were longservice nightworkers Besides, the severe demands as regards the timing in the press industry were never questioned by the men included in this inquiry That is why we did not put the emphasis on the problem of satisfaction Many studies (like Wyatt and Marriott, 1953 ; Akerstedt and Torsvall, 1978) showed that, when a choice is allowed or when economic pressures decrease the extent of shiftwork in a company, a large majority of men prefer day work to night work In the present study, we presume that the results concerning satisfaction would be completely changed if the workers were given the choice between various working schedules, for example if the press company printed a morning and an evening edition In addition one must ask the question of the long-term cost of this adaptation The poor quality of social life is obvious The question becomes: how to live successfully with others when your rest schedules are compelling and out of phase with the environment? There might be a long term biological cost which results in a higher rate of morbidity of these printers when compared with others working in printing companies (Lortie et al , 1978) and perhaps in overmortality as well This latter hypothesis is the topic of a study in progress Of course, night work itself is not the only factor The point is rather to examine the relationship between night schedules and other negative components of the working situation such as noise, toxic exposure and work load.
References Akerstedt, T , Torsvall, L : Experimental changes in shift schedules Their effects on well-being. Ergonomics 21, 849-856 (1978)
Bjerner, B , Holm, A , Swensson, A : Diurnal variations in mental performance: a study of threeshift workers Br J Ind Med 12, 103-110 (1955) Colquhoun, W P , Blake, M J F , Edwards, R S : Experimental studies of shift work I A comparison of "rotating" and "stabilized" four-hour shift systems Ergonomics 11,437-453 (1968 a) Colquhoun, W P , Blake, M J F , Edwards, R S : Experimental studies of shift work II.
Stabilized eight-hour shift systems Ergonomics 11, 527-546 (1968 b) Colquhoun, W P , Blake, M J F , Edwards, R S : Experimental studies of shift work III. Stabilized twelve-hour shift systems Ergonomics 12, 865-882 (1969) Conroy, R T W M , Elliott, A L , Mills, J N : Circadian rhythms in plasma concentration of 11-Hydroxycorticosteroids in men working in night shift and in permanent night workers. Br J Med Psychol 27, 170-174 (1970) Debry, G , Bleyer, R : Influence du rythme des 3 x 8 sur l'alimentation des travailleurs In:
Alimentation et travail, G Debry, R Bleyer (eds ), pp 153-172 Paris: Masson 1972 Evans, F J , Cook, M R , Cohen, H D , Orne, E C , Orne, M T : Appetitive and replacement naps: EEG and behavior Science 197, 687-689 (1977) Foret, J : Sommeil et horaires de travail irr6guliers Dissertation, Lille 1973 Foret, J : L'homme en activity et son sommeil Le Travail Humain 41, 43-54 (1978)
Circadian Rhythms of Permanent Nightworkers
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Foret, J , Benoit, O : Structure du sommeil chez des travailleurs a horaires alternants Electroencephalogr Clin Neurophysiol 37, 337-344 (1974) Foret, J , Benoit, O : Etude du sommeil de travailleurs horaires alternants Adaptation et recuperation dans le cas de rotation rapide de poste ( 3-4 jours) Eur J Appl Physiol 38, 71-82 (1978) Knauth, P , Rutenfranz, J : Untersuchungen zum Problem des Schlafverhaltens bei experimenteller Schichtarbeit Int Arch Arbeitsmed 30, 1-22 (1972) Lortie, M , Teiger, C , Laville, A : Conditions de travail, 6tat de sante et mortality chez des travailleurs de nuit permanents (rotativistes de presse) Laboratoire de Physiologie du Travail, CNAM (in press) Mott, P E , Mann, F C , McLoughlin, Q , Warwick, D P : Shiftwork Ann Arbor: University of Michigan Press 1965 Nachreiner, R , Frielingsdorf, R , Romahn, R , Knauth, P , Kuhlmann, W , Klimmer, R , Rutenfranz, J , Werner, E : Schichtarbeit bei kontinuierlicher Produktion Bundesanstalt fir Arbeitsschutz und Unfallforschung Dortmund, 1975 Patkai, P , Akerstedt, T , Pettersson, K : Field studies of shiftwork I Temporal patterns in psychophysiological activation in permanent night workers Ergonomics 20, 611-619 (1977) Reinberg, A , Migraine, C , Apfelbaum, M , Brigant, L , Ghata, J , Vieux, N , Laporte, A : Circadian and ultradian rhythms in the feeding behavior and nutrient intake of oil refinery operators on a rapidly rotating shift system Ergonomics 21, 862 (1978) Rutenfranz, J , Colquhoun, W P , Knauth, P , Ghata, J : Biomedical and psychosocial aspects of shift work Scand J Work Environ Health 3, 165-182 (1977) Teleky, L : Problems of night work Influences on health and efficiency Ind Med Surg 12, 758-779 (1943) Tune, G S : Sleep and wakefulness in a group of shiftworkers Br J Ind Med 26, 54-58 (1969) Ulich, E : Zur Frage der Belastung des arbeitenden Menschen durch Nacht und Schichtarbeit. Psychol Rundschau 8, 42-61 (1957) Ulich, E : Schicht und Nachtarbeit im Betrieb K 61n, Opladen: Westdeutscher Verlag 1964 Webb, W B : Sleep behavior as a biorhythm In: Biological rhythms and human performance, W P Colquhoun (ed ), pp 149-178 London: Academic Press 1971 Wyatt, S , Marriott, R : Night work and shift changes Br J Ind Med 10, 164-172 (1953) Received November 21, 1978 / Accepted March 8, 1979
