Biological Psychology 5 (1977) 179-190 © North-Holland Publishing Company


Department of Human Sciences, University of Technology, Loughborough, Leicestershire LEl l 3TU, England and OLOV OSTBERG *

Department of Occupational Health, National Board of Occupational Safety and Health, Fack, S-100 26 Stockholm, Sweden Accepted for publication 30 November 1976

Research into individual differences in circadian rhythms is reviewed, particularly mornlngness-eveningness. It was hypothesised that extraverts would be inclined towards eveningness and introverts towards morningness. Forty-eight subjects took regularly their oral temperature. Peak times were identified from smoothed temperature curves. Results showed that extraverts had a peak time insignificantly later than introverts. Re-grouping of the data into the mornlngness-eveningness dimension, based upon the results of a self assessment questionnaire, showed that evening types had significantly later peak times than morning types. Morningness-eveningness was not significantly correlated with extraversion-introversion, although there was a trend. No significant differences were found for sleep lengths with either groupings, or for sleep-wake habits within extraversion-introversion. Morning types retired and arose significantly earlier than evening types. Although sleep-wake habits and extraversion-introversion help to determine peak times there are other contributory factors to peak time which appear to be partly covered by the questionnaire.

1. Introduction Whilst one of the earliest reports of individual differences in circadian rhythms 1 was b y Jiirgensen (1873) who categorised people into those with a marked rhythm and those without, the first questionnaire designed to determine individual differences in these rhythms appears to have been produced by O'Shea (1900) and incorporated such questions as, "During what hours of the day are you at your best?" and, "When are you dullest?". Later, Marsh (1906) used this questionnaire and * Present address to which requests for reprints should be sent: Department of Human Work Sciences, University of Lule~, S-95187 Lule~, Sweden. I Although the term 'circadian' was only introduced by Halberg in 1959 and has only recently come into regular usage, the term will be used to refer to the work of early studies. 179


J.A. Home and O. O'stberg /Individual differences in human circadian rhythms

from the results proposed that 'morning workers' and 'evening workers' could be identified. More specifically, Jundell (1904) considered that the shape of the circadian curve depended upon the degree of 'Morningness' and 'Eveningness' of an individual. A greater orientation towards the consequence of sleeping habits upon morningness-eveningness became apparent with the work of Wuth (1931) and of Winterstein (1932) who differentiated 'evening sleepers' (early to bed, early to rise) and 'morning sleepers' (late to bed, late to rise). IAopold-IAvi (1932) had a more extended typology which also included early to bed, early to rise, and late to bed, early to rise individuals. He believed that the predisposing factor was a dominance of either the sympathetic or para-sympathetic nervous systems. In a very thorough review of individual differences in circadian rhythms incorporating sleeping habits, Freeman and Hovland (1934) proposed that the circadian curves could be divided into one of four types; (1) the continuous rise, (2) the continuous fall, (3) morning rise; afternoon fall, and (4) morning fall; afternoon rise. Kleitman (1939) rejected this classification scheme as having 'too many niches'. Kleitman did not allow for more than two major types of people: those who have temperature and performance peaks early in the day and those who peak much later. The former were called 'Morning types' and the latter 'Evening types'. There was also an intermediate class of 'Afternoon types', but this category was of minor importance. Later, when Kleitman (1949) himself reviewed the literature it was explained that the four types of curve of Freeman and Hovland were the result of employing too small groups of subjects, with the groups being dominated by either morning or evening types and the studies being carried out predominantly either before or after 3 p.m. Some recent investigators, for example Folkard (1975),have proposed that individual differences in daily peak times of performance are not very evident. Folkard extends Kleitman's critiscisms of Freeman and Hovland to also include Kleitman's own work, by proposing that individual differences were only apparent because comparisons were made mostly between the peak times of different types of psychological performance tasks, rather than intra-test comparisons. A revival in the application of morningness--eveningness is now evident and several morningness-eveningness questionnaires have recently been developed. Oquist (1970) produced a Swedish language questionnaire which appeared to be able to distinguish between these two extremes. Ostberg (1973a) modified this questionnaire for use in a circadian rhythm study of food intake and oral temperature, and concluded that the questionnaire did distinguish between the two types and that there are morning type-evening type differences in patterns of food intake and oral temperature. Ostberg (1973b) re-designed the questionnaire for a shiftwork study. The questionnaire had 14 questions and Ostberg claimed that this Swedish questionnaire had the ability to discern individual differences for suitability to shift work. He concluded that morning types did not adapt their habits to the need of shift work as

J.A. Home and O. Ostberg / Individual differences in human circadian rhythms


readily as evening types and that morning types have a more autonomous circadian rhythm. Home and Ostberg (1976) produced an English language morningnesseveningness questionnaire and found that morning types had a significantly earlier circadian peak time of oral temperature than evening types, and also tended to have a higher daytime temperature and a lower postpeak temperature. Although peak times were significantly correlated with bed time and arising time the questionnaire had a better predictive ability for peak time, suggesting that there is more to morningness-eveningness than sleep/wake habits. The dividing up to people according to their rhythm strength or degree of morningness-eveningness has been related to personality theories. In Sheldon's (1942) classification of temperaments the viscerotonics were long sleepers (and possibly morning types), the somatotonics were clearcut morning types, and the cerebrotonics were evening types (and possibly short sleepers). Oquist (1970) investigated the relationships between the personality factors of Cattell's 16 PF and morningness-eveningness and found co-variations between morningness and schizothymia and between eveningness and cyclothymia. Extraverts and introverts have been shown to have different circadian characteristics. Colquhoun (1960) studied visual inspection efficiency in introverts and extraverts. At I0 a.m. the introverted subjects tended to be more efficient than the extraverted subjects and at 3 p.m. the reverse was so. Later, Blake (1967) showed that extraverts and introverts differ slightly in their circadian rhythms of body temperature. Blake's main finding was that extraverts tended to have, on average, a later peak than introverts, but as his graphs were made up of two-point running means no definite conclusion can be drawn about the absolute time displacement between the peak times. These results might be seen to support the viewpoint of Eysenck (1967) that extraverts, being more inclined towards the night life, would tend to reach an activity peak later than introverts. It is possible, therefore, that introverts are more inclined to be the 'early to bed, early to rise' type of individual than extraverts, and the question arises as to whether morning and evening types are related to the extraversion scale, with the hypothesis being that extraverts are more inclined to be evening types, and introverts morning types. The purpose of this study was to investigate this hypothesis by re-categorising the data from the Home and Ostberg (1976) morningness-eveningness study into the extraversion-introversion dimension, and to see whether extraversion-introversion was related to circadian peak time, sleep/wake habits and morningnesseveningness. 2. Method 2.1. Morningness-eveningness

This was determined by means of the Morningness-Eveningness self-assessment questionnaire presented and described in detail elsewhere (Horne and Ostberg,


J.A. Home and O. Ostberg / Individual differences in human circadian rhythms

1976). Briefly, the questionnaire asks 19"questions, selected to assess morningnesseveningness. Most of the answers are forced choice, with no 'do not know/cannot decide' category. Four choices of answer are given, indicating: definite morning type, moderate morning type, moderate evening type, and defmite evening type. A time scale is employed for a few questions. Each question is given a loading factor determined from item analysis. The scores are added and the sum converted into a five point morningness-eveningness scale: Score Definitely morning type 70-86 Moderately morning type 59-69 Neither type 42-58 Moderately evening type 31--41 Definitely evening type 16-30 2.2. Extraversion-Introversion

This was assessed by means of the Eysenck Personality Inventory (EPI). 2.3. Subjects

Forty-eight subjects within the age range 18-32 years, divided approximately equally between the sexes, were paid to take part in the study. It was found from the completed Morningness-Eveningness Questionnaire that the sample consisted of 18 moderate to definite morning types, 20 moderate to definite evening types, and 10 intermediate types. From the EPI scale the subjects consisted of 16 extraverts (scoring 15 or more) 13 introverts (scoring 9 or less) and 19 ambiverts (scoring between 10 and 14 inclusive). Subjects were also required to log carefully their bed time, arising time and any nap times for each day throughout the three weeks of the study. 2.4. The circadian variation o f oral temperature as an external criterion

Individual differences in the circadian rhythm of oral temperature were taken as an external criterion of both the Morningness-Eveningness Questionnaire and of possible circadian differences between extraverts and introverts. Subjects took regularly and logged their oral temperature, using mercury-in-glass-thermometers, at approximately half hour intervals for each day throughout a three week period during the early summer (British Summer Time: BST). Measurement commenced upon awakening and ended at bedtime. Measurements were not taken during sleep. A high daily sample rate over many days was necessary to average out: (a) error of free measurement inherent with mercury-in-glass thermometers, (b) daily individual fluctuations within the trend of oral temperature. Subjects retained their own thermometer. They were carefully trained in reading thermometer scales and were instructed to place the thermometer well under the

J.A. Home and O. Ostberg / Individual differences in human circadian rhythms


tongue with mouth shut for five minutes. Measurements were not taken (1) during eating, drinking, smoking, changing environment, and within 20 minutes of completing these activities, (2) during exercise, and within 2 hours of completing exercise. Subjects were not restricted in their normal daily activities. At the end of the period an averaged waking oral temperature change was compiled for each subject by dividing up the waking day into ¼ hour epochs, making about sixty epochs in all, and averaging all measurements taken over the three week period which fell into each epoch. On average these epochs each contained four readings. In order to smooth the averaged waking oral temperature curve for each subject and to objectively identify the peak time a curve fitting technique had to be employed. The commonly accepted least squares fitting of a cosine model, the 'cosinor analysis', was not seen to be suitable in this instance. The reasons being that, firstly, the temperature data were not available for the complete 24 hour period as data were not collected during sleep. Secondly, Harris (1974) has noted that the cosinor analysis is not very sensitive to skewness, and as the present study was particularly interested in individual differences in temperature curves and possible differences in skewness, this method of analysis would be inappropriate. Instead, the method of least squares fitting of polynomials was considered to be more suitable and a polynomial curve fitting program (BMDO5R-1966-Health Sciences Computing Facility, UCLA), using the sextic term, was employed. Peak times were identified from each subject's polynomial curve. The temperature for every hour from 09.00-23.00 hours was assessed from each curve and

3ZO. ,9, ,8. ,7. ,6.

°c.5. ,4. ,3.

Deviation For Hourly


• Introvert {E.P.I.,9.N 13)


• Ambiverts(N 19) v Extrovert (E.P.I.~I$.N 16)

Means I



06 o'7 de ~

1'o 11 1'2 13 14 1'5 1'6 Ir

l's ;9 ~

2'1 2'2 2'3 2'+

Hours Summer ILS.T.

Fig. 1. Mean daytime oral temperature curves for 3 extraversion groups based on E.P.[. scores

(N = 48).

J.A. Horne and O. Ostberg ~Individual differences in human circadian rhythms



37,0. ,9. ,8.





.7. ,6.

°C ,5.~



,4. ,3_

• Morning Type (1'418) • Intermediate Type ( N 10) v Evening Type (N 20)

,2. J.

t Average Standard Deviation For Hourly Means(All Types)


06 0'7 0'8 09 I'0 I'I 1'2 1'3 I~I I'5 I'6 I'7 I~ (9 20 2'I 2'2 2'3 24 Hours SummerB.S.T.

Fig. 2. Mean daytime oral temperature curves for morning & evening types based upon questionnaire (N = 48).

pooled for, (a) each of the morning, evening and intermediate groupings. (b) each of extravert, introvert, and ambivert groupings. Data were pooled for each of the hourly points and for peak times in respect of (a) and (b). Both for the hourly points and for the peak times unrelated 't tests' were performed with pairs of groups taken from within (a) and (b). The smoothed temperature curves were pooled for each of the categories in these two groupings and are shown in figs. 1 and 2. To assess whether or not any differences in peak times between the three categories of each grouping might be related to differences in sleep-waking patterns, the temperature logs of each subject were scrutinised for bed times and arising times. Each of these times was averaged for the categories within (a) and (b). Subjective estimates of sleep length were also taken. Spearman rank order correlations were calculated between pairs of dimensions taken from: morningness-eveningness, extraversion-introversion, oral temperature peak time, bed tin]e, arising time and sleep length.

3. Results

Fig. 1 shows that the average oral temperatures upto the peak times tend to be higher for introverts than for extraverts, with the reverse trend after the peak

J.A. Home and O. Ostberg / Individual differences in human circadian rhythms


Table I Sleep parameters and temperature peak times for Extraverts, Introverts and Ambiverts.

Bed time Mean (BST) s.d. (min) Arising time Mean (BST) s.d. (min) Sleep length Mean (min) s.d. (min) Peak time Mean (BST) s.d. (min)

Extraverts (iV = 16)

Ambiverts (N = 19)

Introverts (N = 13)

00.12 hr 80

00.17 hr 60

23.50 hr 53

08.17 hr 67

08.28 hr 93

08.14 hr 57

439 55

457 51

436 41

20.25 hr 111

20.13 hr 118

19.52 hr 97

time. However the standard deviations for each hourly point were fairly large, averaging 0.13°C, especially for the ambivert group. This factor, together with a relatively small difference between the average curves, produced no statistically significant between group differences for the points. Although extraverts tended, on average, to have peak times 33 min later than introverts, it can be seen from table 1 that the variance around these peak times was large, resulting in an insignificant difference between these times. The average hourly temperatures and the peak time o f the ambiverts tended to be located between those o f the other two groups. Tables 1 and 2 show that although there were trends for extraverts to retire and arise later than introverts, these differences were not significant. From fig. 2 it is apparent that the averaged temperature curves o f the morning, evening and internaediate types tend to show similar trends to the respective groupings of extraverts, introverts and ambiverts, shown in fig. 1. Morning types tend to have a higher temperature upto the peak time than the evening type, with a reversed trend following the peak. Although the standard deviations for each hourly point within each group o f fig. 2 were generally smaller'than for the groupings o f

Table 2 Statistical comparisons of sleep parameters and peak times between Extravert, Introvert and Ambivert groups, using two-tailed t-tests.

Extraverts × Introverts Extraverts X Ambiverts Introverts X Ambiverts

Bed time

Arising time

Peak time

Sleep length

n.s. n.s. n.s.

n.s. n.s. n.s.

n.s. n.s. n.s.

n.s. n.s. n.s.


J.A. Horne and O. Ostberg / Individual differences in human circadian rhythms

Table 3 Sleep parameters and temperature peak times for Morning, Evening and Intermediate types.

Bed time Mean (BST) s.d. (min) •Arising time Mean (BST) s.d. (min) Sleep length Mean (min) s.d. (min) Peak time Mean (BST) s.d. (min)

Evening type (N = 20)

Intermediate type (N = 10)

Morning type (N = 18)

01.05 hr 27

23.30 hr 58

23.26 hr 42

09.18 hr 68

08.07 hr 52

07.24 hr 45

434 47

452 51

454 54

20.40 82

20.25 134

19.32 113

fig. I , they were still relatively large, averaging 0.1°C. Consequently, there were no significant differences between any o f the morningness-eveningness groupings for any o f the hourly points. However, there was a significant difference (p = 0.05) between the peak times o f the morning and evening types, with the former peaking on average 68 min earlier than the latter. This is reflected in the significant differences for bed time (p = 0.001) and arising time (p = 0.001) between morning and evening types, with intermediate types retiring at a similar time to the morning types, but significantly different (p = 0.001) from the evening types. The intermediate type tended to arise at a time between, and significantly different from (p = 0.001) the arising times o f the other two groups. Again, sleep length shows no significant differences between the morning, evening and intermediate types. Although there are trends in the matrix o f table 5 in the correlations between extraversion-introversion and with both morningness--eveningness and with peak times, in the direction o f extraverts tending to be inclined towards eveningness and a later peak time, these do not reach statistical significance. The correlations

Table 4 Statistical comparisons of sleep parameters and peak times between Morning, Evening and Intermediate types, using two-tailed t-tests.

Evening X Morning types Evening X Intermediate types Morning X Intermediate types

Bed time

Arising time

Peak time

Sleep lenth

0.001 0.001 n.s.

0.001 0.001 0.001

0.05 n.s. n.s.

n.s. n.s. n.s.

J.A. Home and O. Ostberg /Individual differences in human circadian rhythms


Table 5 Correlations of measures with significancelevels using Spearman rank order correlations.

1. Morningness-Eveningness 2. Peak time 3. Bed time 4. Arising time 5. Sleep length 6. Extraversion- Introversion






-0.51 ** -0.67 *** -0.79 *** +0.07 -0.22

+0.37 * +0.42 ** -0.13 +0.29

+0.65 *** -0.16 -0.02

-0.08 -0.06



*** Significant at p:O.O01 (two-tail) ** Significant at p:O.O1 (two-tail) * Significant at p:O.05 (two-tail) between extraversion-introversion and with bed time, arising time and sleep length are low and without obvious trends. Morningness-eveningness significantly correlated with peak time (p - 0.51 ;p = 0.01) and with bed time (p - 0.67; p = 0.001) and arising time (p - 0.79; p = 0.001). Although both bed time and arising time significantly correlated with peak time (p + 0.37; p = 0.05) (p + 0.42; p = 0.01) they have a lower predictive ability of peak time than does morningness-eveningness. As might be expected, bed time and arising time are significantly correlated (p + 0.65; p = 0.001). Sleep length is not significantly correlated with any of the other variables.

4. D i s c u s s i o n

Comparison of Blake's (1967) findings with those of the present study reveal an obvious similarity between the two sets of results. Both show similar peak times for each of the extravert and introvert groups, a slightly higher daytime temperature upto the peak for introverts and a post-peak reversal of this trend. Whilst Blake and Corcoran (1972) reported that introverts had a temperature peak time at least an hour earlier than extraverts the present study found this difference to be only 33 min. Blake and Corcoran considered that the curve differences between extraverts and introverts was due to a phase shift, and it would appear that a similar conclusion can be made with the present extraversion-introversion temperature data. However, from both the peak time and the correlation data of the present study it appears that although there is a tendency for extraverts to be evening types and introverts morning types a more significant differentiation of the peak times of these subjects can be attained though the morningness--eveningness dimension. Although extraversion-introversion might appear to play a minor role in morningness-eveningness there is clearly much more to momingness-eveningness than extraversion-introversion. Therefore the hypothesised relationship between extra-


J.A. Home and O. Ostberg / Individual differences in human circadian rhythms

version-introversion and morningness-eveningness is not substantiated statistically. The present findings of insignificant differences in sleep-wake habits between extraverts and introverts is in accordance with Tune's (1969) data for 20-29 year olds. But, Tune also found that sleeping habits are to some extent dependent upon the interaction between age and extraversion-introversion. Although he found no significant differences in sleep-wake habits between extraverts and introverts in this younger age group, there were clear differences for older subjects. With increasing age above 40 years the extraverts slept progressively longer and awoke later, and in effect became more and more like evening types, whilst the introverts slept shorter and effectively became more like morning types. Thus, from the work of Tune it is possible that a higher and perhaps significant correlation between extraversion-introversion and morningness-eveningness may develop in middle age. However, it must be noted that Tune used subjective reports of times of falling asleep and awakening, whereas the present study employed bed and arising times. It is well known amongst sleep researchers that sleep estimates such as those obtained by Tune are not always reliable, and for this reason it is felt that 'bed time' and 'arising time' might be more valid estimates of sleep-wake habits. Unlike the present study, Tune included daytime naps in his daily sleep length data, but over the entire eight week sampling period used by Tune each individual in the 20-29 years age group took a daytime nap on an average of only 0.5-2.0 occasions. The incidence of daytime naps in the present study was almost negligable, averaging 0.2 naps per person over the entire three.week sample period. From the morningness-eveningness data of fig. 2 and tables 3 and 4 some interesting trends can be seen. Although the average morning type appears to start the day nearly two hours earlier than the evening type the overall higher temperature curve upto the peak time for the former group appears not be be merely an advancement on the latter by two hours. There is some indication that whilst the morning type displays a relatively rapid waking temperature rise, culminating in a plateau which is terminated in a slight but obvious peak at about 19.30 hr, the evening type tends to display a steady temperature rise throughout the day eventually reaching a distinct peak of similar amplitude but about 70 min later than that of the morning type. After the peaks have been reached the rates of temperature decline for both groups appear to be similar, but with the evening type lagging behind the morning type by about an hour, leading to a difference in bed time of about I~A hours. Whilst the difference in peak times between the two groups is probably related to the evidence that morning types have a sleep-wake life style about an hour in advance of evening types, the dynamics of the temperature curves of the two groups from arising to peak times appears not just to be a reflection of such a phase lag. Even though bed time and arising time are significantly correlated with peak time, squaring these correlations to obtain the variance shows that only a minority of the total variance of peak times can be accounted for by either bed time or arising time. Thus there appears to be more to the determination of peak time than sleep-wake habits.

J.A. Home and O. Ostberg / Individual differences in human circadian rhythms


However, it must be noted that these subjects are not necessarily typical of a normal population, particularly in respect of life style. For example from table 3 the arising time for these students, particularly the evening types, would seem to be later than an arising time which could be expected from an older population having to start a regular job of work at 07.00-09.00 hr with various family commitments prior to work. The question of whether or not the apparent afternoon temperature dip of the intermediate group in fig. 2 can be equated with the post-lunch dip in performance reported by Blake (1967) must at present remain unanswered, particularly as the present temperature data for this group were only made up of ten subjects. However, this is an area worthy of further investigation, especially as Hockey and Colquhoun (1972) point out in their review that although there is support for a post lunch dip in performance there is no evidence of an accompanying temperature dip. From the present study further research issues can now be raised, for example, whether aspects of personality other than extraversion-introversion might relate to morningness-eveningness, and whether the degree of morningness-eveningness for any individual is a constant or labile phenomenon throughout life. Unfortunately, there is little guidance over these matters from the literature and these questions must remain unanswered at present. It must also be noted that although the peak time has been used as a reference point in this study it can vary within individuals who lead a fairly regular life. For example, the magnitude of the peak-time differences between morning and evening types may be influenced by seasonal changes in the circadian peak-time of body temperature. Horne and Coyne (1975) found in a study conducted in the climatic and environmental conditions of the British Isles, that the circadian peak time of oral temperature became delayed by about an hour from December to June. This change was not related to clock time changes. Thus, there may be a seasonal interaction between temperature peak times and morningness-eveningness.

Acknowledgement At the time of this research O. Ostberg was in receipt of a Leverhulme Travelling Fellowship, tenable at Loughborough University.

References Blake, M.J.F. (1967). Relationship between circadian rhythm of body temperature and introversion-extraversion. Nature, 215,896-897. Blake, M.J.F. and Corcoran, D.W.J. (1972). Introversion-extraversion and circadian rhythms. In: Colquhoun, W.P. (Ed.) Aspects of Human Efficiency, London: English UniversityPress, 261-272.


J.A. Home and O. Ostberg / lndivMual differences in human circadian rhythms

Colquhoun, W.P. (1960). Temperament, inspection efficiency and time of day. Ergonomics, 3, 377-378. Eysenck, H.J. (1967). The Biological Basis of Personality. Illinois: C.C. Thomas. Folkard, S. (1975). Diurnal variation in logical reasoning. British Journal of Psychology, 66, 1-8. Freeman, G.L. and Hovland, C.I. (1934). Diurnal variations in performance and related physiological processes. Psychological Bulletin, 31,777-799. Harris, E.K. (1974). Comments on statistical methods for analysing biological rhythms. In: Scheving, L.E., Halberg, D.F. and Pauly, J.E. (Eds.) Chronobiology. Tolyo: Igaku Shoin Ltd., 757-760. Hockey, G.R.J. and Colquhoun, W.P. (1972). Diurnal variation in human performance: a review. In: Colquhoun, W.P. (Ed.), Aspects of Human Efficiency, London: English University Press, 1-24. Home, J.A. and Coyne, I. (1975). Seasonal changes in the ~ircadian variation of oral temperature during wakefulness. Experientia, 31, 1296-1297. Horne, J.A, and Ostberg, O. (1976). A self-assessment questionnaire to determine Morningness-eveningness, International Journal of Chronobiology, 4, 97-110. Jundell, I. (1904). Uber die nykthermeralen Temperaturschwankungen im ersten Lebensjahre des Menschen. Jahrbuch f0r Kinderheilkunde, 59, 521-619. Jiirgensen, T. (1873), Die KiSrperw/irmedes gesunden Menschen. Leipzig: Fogel. Kleitman, N. (1939). Sleep and Wakefulness, Chicago: University of Chicago Press. (Revised and enlarged edition 1963). Kieitman, N. (1949). Biological rhythms and cycles. Physiological Review, 29, 1-30. L6opold-L6vi, M. (1932). Le lever matinual pr6coce. Bulletin et m6moires de la Societ6 de M6decine de Paris, 117-121. Marsh, H.D. (1906). The diurnal course of efficiency. Contributions to Philosophy and Psychology from the University of Columbia, 14, No. 3. Oquist, O. (1970). Kartl/iggning av individualla dygnsrytmer. Thesis at the Department of Psychology, University of Goteborg, Sweden. O'Shea, M.V. (1900). Aspects of mental economy. Bulletin of the University of Wisconsin, 2, 33-198. Ostberg, O. (1973a). Circadian rhythm of food intake and oral temperature in "morning" and "evening" groups of individuals. Ergonomics, 16,203-209. Ostberg, O. (1973b). Interindividual differences in circadian fatigue patterns of shift workers. British Journal of Industrial Medicine, 30, 341-351. Sheldon, W.H., (1942). The Varieties of Temperament. New York: Harper and Brothers. (4th ed.) Tune, G.S. (1969). The influence of age and temperament on the adult human sleep-wakefulness patterns. British Journal of Psychology, 60, 431-441. Winterstein, H. (1932). Schlaf u nd Traum. Berlin: Springer. Wuth, O. (1931). Klinik und Therapie der Schlafstt~rungen. Sehweizeriche Medizinische Wochenschrift, 12, 833-837.

Individual differences in human circadian rhythms.

Biological Psychology 5 (1977) 179-190 © North-Holland Publishing Company INDIVIDUAL DIFFERENCES IN HUMAN CIRCADIAN RHYTHMS JAMES A. HORNE Departmen...
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