Regression Models for the Estimation of Orcadian Rhythms in the Presence of Effects Due to Masking

The estimation of human circadian rhythms from experimental data is complicated by the presence of “masking” effects associated with the sleep-wake cycle. The observed rhythm may include a component due to masking, as well as the endogenous component linked to a circadian pacemaker. In situations where the relationship between the sleep-wake cycle and the circadian rhythm is not constant, it may be possible to obtain individual estimates of these two components, but methods commonly used for the estimation of circadian rhythms, such as the cosinor analysis, spectral analysis, average waveforms and complex demodulation, have not generally been adapted to identify the modulations that arise from masking. The estimates relate to the observed rhythms, and the amplitudes and acrophases do not necessarily refer to the endogenous rhythm.

In this paper methods are discussed for the separation of circadian and masking effects using regression models that incorporate a sinusoidal circadian variation together with functions of time since sleep and time during sleep. The basic model can be extended to include a time-varying circadian rhythm and estimates are available for the amplitude and phase at a given time, together with their joint confidence intervals and tests for changes in amplitude and acrophase between any two selected times. Modifications of these procedures are discussed to allow for non-sinusoidal circadian rhythms, non-additivity of the circadian and time-since-sleep effects and the breakdown of the usual assumptions concerning the residual errors.

This approach enables systematic masking effects associated with the sleep-wake cycle to be separated from the circadian rhythm, and it has applications to the analysis of data from experiments where the sleep-wake cycle is not synchronized with the circadian rhythm, for example after time-zone transitions or during irregular schedules of work and rest.     

The Shape of the Endogenous Circadian Rhythm of Rectal Temperature in Humans

Fourteen healthy subjects have been studied in an isolation unit while living on a 30h “day” (20h awake, 10h asleep) for 14 (solar) days but while aware of real time. Waking activities were sedentary and included reading, watching television, and so forth. Throughout, regular recordings of rectal temperature were made, and in a subgroup of 6 subjects, activity was measured by a wrist accelerometer. Temperature data have been subjected to cosinor analysis after “purification,” a method that enables the endogenous (clock-driven) and exogenous (activity-driven) components of the circadian rhythm to be assessed. Moreover, the protocol enables effects due to the circadian rhythm and time-since-waking to be separated. Results showed that activity was slightly affected by the endogenous temperature rhythm. Also, the masking effects on body temperature exerted by the exogenous factors appeared to be less than average in the hours before and just after the peak of the endogenous temperature rhythm. This has the effect of producing a temperature plateau rather than a peak during the daytime. The implications of this for mental performance and sleep initiation are discussed. (Chronobiology International, 13(4), 261-271, 1996)     

Different Behavior of the Circadian Rhythms of Activity and Body Temperature During Resynchronization Following an Advance of the LD Cycle

Spontaneous activity and the body temperature of laboratory mice were recorded telemetrically using implantable transmitters. Following ten control days (L : D = 12 : 12; light from 07:00 to 19:00), the LD cycle was phase-advanced by shortening the light time by 8 h. Recordings were continued for a further 3 weeks. The raw temperature data were unmasked or 'purified' — that is, the temperature changes due to locomotor activity were removed, so revealing the endogenous component of the rhythm — using a regression method previously developed by us. The circadian rhythms of activity and measured body temperature resynchronized on average after 8 days. During resynchronization, both rhythms tended to show two components, one adjusting by a phase advance and the other by a phase delay. However, after purification of the body temperature rhythm, only the advancing component remained. These results indicate that the delaying component of the measured temperature rhythm was caused by masking due to activity, and that the endogenous component of this rhythm did not divide into two components during the resynchronization process. Also, the endogenous component of the circadian rhythm of body temperature and one component of the activity rhythm seemed to be controlled by the same oscillator. It remains uncertain how the other component of the activity rhythm is regulated.     

Effect of Chronic Caloric Restriction on the Circadian Regulation of Physiological and Behavioral Variables in Old Male B6C3F1 Mice

The circadian rhythms of food and water consumption, the number of feeding and drinking episodes, oxygen consumption, carbon dioxide production, respiratory quotient, gross motor activity, and body temperature were measured in male B6C3F, mice that were fed ad libitum (AL) or fed a caloric-restricted diet (CR). The CR regimen (60% of the normal AL consumption) was fed to mice during the daytime (5 hr after lights on). CR animals exhibited fewer feeding episodes but consumed more food per feeding bout and spent more total time feeding than AL mice. It appears that CR caused mice to change from their normal “nibbling behavior” to meal feeding. Compared to AL animals, the mean body temperature was reduced in CR animals, while the amplitude of the body temperature rhythm was increased. Spans of reduced activity, metabolism, and body temperature (torpor) occurred in CR mice for several hours immediately before feeding, during times of high fatty acid metabolism (low RQ). The acute availability of exogenous substrates (energy supplies) seemed to modulate metabolism shifting metabolic pathways to promote energy efficiency. CR was also associated with lower DNA damage, higher DNA repair, and decreased proto-oncogene expression. Most of the circadian rhythms studied seemed to be synchronized primarily to the feeding rather than the photoperiod cycle. Night-time CR feeding was found to be better than daytime feeding because the circadian rhythms for AL and CR animals were highly synchronized when this regimen was used.     

The Effect of Activity on the Waking Temperature Rhythm in Humans

Nine healthy female subjects were studied when exposed to the natural light-dark cycle, but living for 17 “days” on a 27h day (9h sleep, 18h wake). Since the circadian endogenous oscillator cannot entrain to this imposed period, forced desynchronization between the sleep/activity cycle and the endogenous circadian temperature rhythm took place. This enabled the effects of activity on core temperature to be assessed at different endogenous circadian phases and at different stages of the sleep/activity cycle. Rectal temperature was measured at 6-minute intervals, and the activity of the nondominant wrist was summed at 1-minute intervals. Each waking span was divided into overlapping 3h sections, and each section was submitted to linear regression analysis between the rectal temperatures and the total activity in the previous 30 minutes. From this analysis were obtained the gradient (of the change in rectal temperature produced by a unit change in activity) and the intercept (the rectal temperature predicted when activity was zero). The gradients were subjected to a two-factor analysis of variance (ANOVA) (circadian phase/ time awake). There was no significant effect of time awake, but circadian phase was highly significant statistically. Post hoc tests (Newman-Keuls) indicated that gradients around the temperature peak were significantly less than those around its trough. The intercepts formed a sinusoid that, for the group, showed a mesor (±SE) of 36.97 (±0.12) and amplitude (95% confidence interval) of 0.22°C (0.12°C, 0.32°C). We conclude that this is a further method for removing masking effects from circadian temperature rhythm data in order to assess its endogenous component, a method that can be used when subjects are able to live normally. We suggest also that the decreased effect of activity on temperature when the endogenous circadian rhythm and activity are at their peak will reduce the possibility of hyperthermia.     

The Difference Between Activity When in Bed and Out of Bed. II. Subjects on 27-Hour “Days”

Nine healthy subjects have been studied while exposed to the normal alternation of light and dark, but with their sleep and activity pattern adjusted to a 27-h “day” for 17 imposed “days.” Rectal temperature showed clearly the competing influences of 27-h and 24-h components, and these were separated by the method of “purification.” The method indicated that the endogenous component had a constant amplitude throughout the experiment and remained entrained to solar (24-h) time; by contrast, the exogenous component followed the imposed 27-h “day” and increased rectal temperature in proportion to the amount of subjects' activity. Wrist movement was used to assess activity while in bed (attempting sleep) and out of bed (when naps were forbidden). While these results confirmed adherence of the subjects to the imposed 27-h “days,” they also showed that the dichotomy between “out of bed” activity and “in bed” inactivity depended on the phase relationship between endogenous (24h) and exogenous (27h) components. Thus, the dichotomy was highest and was equal to that during control days (with a conventional 24-h life-style) when the two components were in phase and lowest when the solar and imposed day were in antiphase. This was due to changes in activity, both during time spent in bed and out of bed.

We confirm that this protocol can produce valuable information about the properties of the circadian system in humans and the value of the process of purification of temperature data. We have established also that the very simple and noninvasive measurement of wrist movement, coupled with its use to calculate dichotomy indices, provides valuable information that both confirms and extends the results obtained from the more conventional (butalso more invasive) measurement of rectal temperature.     

The Pragmatic Approach to Masking

This paper advocates the use of a pragmatic approach to the problem of masking in real-life situations involving an abrupt change in the timing of sleep, i.e. shiftwork and “jet-lag” situations. Although “pure” chronobiological research has pointed to the importance of taking masking effects into account, the techniques that it has provided for doing so are extremely difficult to apply in real-life situations. The approach advocated here is based on Wever's pioneering work, and involves estimating the normative endogenous and exogenous components of the circadian rhythm in body temperature. These estimates are then used to: (a) simulate the results of shiftwork studies; and (b) to “remove” the exogenous component in “jet-lag” studies to allow analysis of the estimated endogenous component. The simulated curves obtained cross-correlated extremely highly with published night-shift temperature curves, while the “removal” of the exogenous component resulted in very similar findings to those obtained in temporal isolation studies. It is concluded that this pragmatic approach to masking may prove extremely useful in interpreting the results of field studies of shiftwork and “jet-lag”.     

The Relation of Shift Work Tolerance to the Circadian Adjustment

The amplitude and phasing of circadian rhythms are under discussion as possible predictors of tolerance to night work. In a field study, subjective sleepiness and oral temperature of 147 female nurses were measured at 2-hour intervals during a period with one morning shift and two consecutive night shifts. The nurses also filled out a questionnaire. Two types of tolerance indices were constructed: The “health index” was based on questions referring to general fatigue, gastrointestinal symptoms, and sleep disturbances, and the “sleepiness index” on the actual subjective ratings of sleepiness. According to the health index, the group with good tolerance had a larger circadian amplitude of the oral temperature rhythm on the day of the morning shift than the group with poor tolerance. However, with regard to the sleepiness index, the corresponding difference between the groups with good or poor tolerance was not significant. The data did not confirm the hypothesis that predicts a quick adjustment of the circadian rhythm when the circadian amplitude is small before the change to night work. The contradictory results found in this and in other studies do not yet permit prediction of tolerance to night work.     

Different Behavior of the Circadian Rhythms of Activity and Body Temperature During Resynchronization Following an Advance of the LD Cycle

Spontaneous activity and the body temperature of laboratory mice were recorded telemetrically using implantable transmitters. Following ten control days (L : D = 12 : 12; light from 07:00 to 19:00), the LD cycle was phase-advanced by shortening the light time by 8 h. Recordings were continued for a further 3 weeks. The raw temperature data were unmasked or ‘purified’ — that is, the temperature changes due to locomotor activity were removed, so revealing the endogenous component of the rhythm — using a regression method previously developed by us. The circadian rhythms of activity and measured body temperature resynchronized on average after 8 days. During resynchronization, both rhythms tended to show two components, one adjusting by a phase advance and the other by a phase delay. However, after purification of the body temperature rhythm, only the advancing component remained. These results indicate that the delaying component of the measured temperature rhythm was caused by masking due to activity, and that the endogenous component of this rhythm did not divide into two components during the resynchronization process. Also, the endogenous component of the circadian rhythm of body temperature and one component of the activity rhythm seemed to be controlled by the same oscillator. It remains uncertain how the other component of the activity rhythm is regulated.     

Linear demasking techniques are unreliable for estimating the circadian phase of ambulatory temperature data.

Clinical investigators often use ambulatory temperature monitoring to assess the endogenous phase and amplitude of an individual's circadian pacemaker for diagnostic and research purposes. However, an individual's daily schedule includes changes in levels of activity, in posture, and in sleep-wake state, all of which are known to have masking or evoked effects on core body temperature (CBT) data. To compensate for or to correct these masking effects, many investigators have developed "demasking" techniques to extract the underlying circadian phase and amplitude data. However, the validity of these methods is uncertain. Therefore, the authors tested a variety of analytic methods on two different ambulatory data sets from two different studies in which the endogenous circadian pacemaker was not synchronized to the sleep-wake schedule. In both studies, circadian phase estimates calculated from CBT collected when each subject was ambulatory (i.e., free to perform usual daily activities) were compared to those calculated during the same study when the same subject's activities were controlled. In the first study, 24 sighted young and older subjects living on a 28-h scheduled "day" protocol were studied for approximately 21 to 25 cycles of 28-h each. In the second study, a blind man whose endogenous circadian rhythms were not synchronized to the 24-h day despite his maintenance of a regular 24-h sleep-wake schedule was studied for more than 80 consecutive 24-h days. During both studies, the relative phase of the endogenous (circadian) and evoked (scheduled activity-rest) components of the ambulatory temperature data changed progressively and relatively slowly, enabling analysis of the CBT rhythm at nearly all phase relationships between the two components. The analyses of the ambulatory temperature data demonstrate that the masking of the CBT rhythm evoked by changes in activity levels, posture, or sleep-wake state associated with the evoked schedule of activity and rest can significantly obscure the endogenous circadian component of the signal, the object of study. In addition, the masking effect of these evoked responses on temperature depends on the circadian phase at which they occur. These nonlinear interactions between circadian phase and sleep-wake schedule render ambulatory temperature data unreliable for the assessment of endogenous circadian phase. Even when proposed algebraic demasking techniques are used in an attempt to reveal the endogenous temperature rhythm, the phase estimates remain severely compromised.     

Temporal pattern of LH secretion: regulation by multiple ultradian oscillators versus a single circadian oscillator

The possibility that the 24h rhythm output is the composite expression of ultradian oscillators of varying periodicities was examined by assessing the effect of external continuously or pulsed (20-minute) Gonadotropinreleasing hormone (GnRH) infusions on in vitro luteinizing hormone (LH) release patterns from female mouse pituitaries during 38h study spans. Applying stepwise analyses (spectral, cosine fit, best-fit curve, and peak detection analyses) revealed the waveform shape of LH release output patterns over time is composed of several ultradian oscillations of different periods. The results further substantiated previous observations indicating the pituitary functions as an autonomous clock. The GnRH oscillator functions as a pulse generator and amplitude regulator, but it is not the oscillator that drives the ultradian LH release rhythms. At different stages of the estrus cycle, the effect of GnRH on the expression of ultradian periodicities varies, resulting in the modification of their amplitudes but not their periods. The functional output from the system of ultradian oscillators may superimpose a “circadian or infradian phenotype” on the observed secretion pattern. An “amplitude control” hypothesis is proposed: The temporal pattern of LH release is governed by several oscillators that function in conjunction with one another and are regulated by an amplitude-controlled mechanism. Simulated models show that such a mechanism results in better adaptive response to environmental requirements than does a single circadian oscillator. (Chronobiology International, 18(3), 399-412, 2001)     

A comparison of some different methods for purifying core temperature data from humans

Nine healthy females were studied about the time of the spring equinox while living in student accommodations and aware of the passage of solar time. After 7 control days, during which a conventional lifestyle was lived under a 24h “constant routine,” the subjects lived 17 × 27h “days” (9h sleep in the dark and 18h wake using domestic lighting, if required). Throughout the experiment, recordings of wrist activity and rectal (core) temperature were taken. The raw temperature data were assessed for phase and amplitude by cosinor analysis and another method, “crossover times,” which does not assume that the data set is sinusoidal. Two different purification methods were used in attempts to remove the masking effects of sleep and activity from the core temperature record and so to measure more closely the endogenous component of this rhythm; these two methods were “purification by categories” and “purification by intercepts.” The former method assumes that the endogenous component is a sinusoid, and that the masking effects can be estimated by putting activity into a number of bands or categories. The latter method assumes that a temperature that would correspond to complete inactivity can be estimated from measured temperatures by linear regression of these on activity and extrapolation to a temperature at zero activity. Three indices were calculated to assess the extent to which exogenous effects had been removed from the temperature data by these purification methods. These indices were the daily variation of phase about its median value; the ratio of this variation to the daily deviation of phase about midactivity; and the relationship between amplitude and the square of the deviation of phase from midactivity. In all cases, the index would decrease in size as the contribution of the exogenous component to a data set fell. The purification by categories approach was successful in proportion to the number of activity categories that was used, and as few as four categories produced a data set with significantly less masking than raw data. The method purification by intercepts was less successful unless the raw data had been “corrected” to reflect the direct effects of sleep that were independent of activity (a method to achieve this being produced). Use of this purification method with the corrected data then gave results that showed least exogenous influences. Both this method and the purification by categories method with 16 categories of activity gave evidence that the exogenous component no longer made a significant contribution to the purified data set. The results were not significantly influenced by assessing amplitude and phase of the circadian rhythm from crossover times rather than cosinor analysis. The relative merits of the different methods, as well as of other published methods, are compared briefly; it is concluded that several purification methods, of differing degrees of sophistication and ease of application to raw data, are of value in field studies and other circumstances in which constant routines are not possible or are ethically undesirable. It is also concluded that such methods are often somewhat limited insofar as they are based on pragmatic or biological, rather than mathematical, considerations, and so it is desirable to attempt to develop models based equally on mathematics and biology. (Chronobiology International, 17(4), 539-566, 2000)     

Temporal structuring of delivery in the absence of a photoperiod: preparturient myometrial activity of the rhesus monkey is related to maternal body temperature and depends on the maternal circadian system.

No convincing evidence exists that the shift from myometrial contractures to contractions, which determines the synchronized 24-h rhythm in the dynamics of the primate uterus, may be attributed to an endogenous circadian rhythm. We therefore wished to ascertain whether a 24-h periodic shift would also occur in the myometrial activity of animals kept under constant conditions. We studied five pregnant rhesus monkeys, kept in continuous darkness from 56-77 days gestational age until delivery at 117-167 days gestational age. During the last week before delivery we determined the individual phase, level, and amplitude of circadian changes in maternal body temperature and 24-h myometrial activity patterns in the form of contractions. In all five monkeys, a rhythm with a period of 24-h characterized the temporal incidence of preparturient contraction activity. A consistent phase lag of 6-7 h from the temperature crest was observed in four out of the five animals. The circadian phase of all individual rhythms was idiosyncratic among animals. We conclude that endogenous rhythms in body temperature and preparturient myometrial activity are truly circadian. In addition, these rhythms are either interdependent or subject to the same maternal timekeeping mechanism, supporting the hypothesis that the exact time of the day at which birth occurs in the rhesus monkey depends on the maternal circadian system.     

Human Circadian Time Structure in Subjects of Different Gender and Age

Most human variables exhibit rhythms with an about 24 hour (circadian) period. Each rhythm can be characterized by its acrophase (calculated peak time of the cosine curve best fitting to the data), its amplitude and rhythm adjusted mean (MESOR). The sequential array of the rhythms' acrophases represents the temporal order of the human time structure. In the present work we used circadian rhythms of 24 chemical and 15 hormonal variables extracted from published studies which were done in a defined area of southeastern Europe (Romania). All studies had a comparable experimental design and were analyzed biochemically and statistically in the same laboratory. The acrophases of these rhythms obtained from both genders of different age groups (from the 2nd to the 9th decade of age) were subjected to multiple correlation test, cluster and principal coordinates analyses. The results show that the temporal order is affected both by gender and age, and evaluate the degree of the effect, offer a “chronbiologic fingerprint” for the examined groups and assist in dissecting rhythm variability among populations.     

Chronophysiology of the cardiovascular system

The development of ambulatory blood pressure monitoring devices and the beat-by-beat measurement of heart rate have enabled it to be established that there are circadian rhythms in heart rate and blood pressure in subjects living normally. Investigations of these variables have led to quantification of their fall at night, and rapid rise on awakening and becoming active in the morning. These changes are of particular interest insofar as abnormalities in them are associated with cardiovascular problems and morbidity in patients and also act as risk factors in otherwise healthy individuals. It has also been shown that there are many other variables of the cardiovascular system. The causes of the circadian rhythms in heart rate and blood pressure are outlined, with particular stress upon the role of the autonomic nervous system, as assessed from low- and high-frequency components of the variation in heart rate measured beat-by-beat. Activity increases blood pressure, but there is evidence that this “reactivity” varies with time of day, and this also might be related to cardiovascular morbidity. Based upon data from several sources, including night work, resting subjects and bed-ridden patients, it is concluded that the contribution of the “body clock” to producing the circadian rhythm in heart rate and blood pressure is relatively small. A bias towards an exogenous cause applies also to most other circadian rhythms in the cardiovascular system. Knowledge of circadian rhythmicity in cardiovascular system, together with an understanding of its causes, provides a rationale for advice to reduce cardiovascular risk and to assess the efficacy of therapies.     

The development of new purification methods to assess the circadian rhythm of body temperature in Mongolian gerbils

Six Mongolian gerbils were studied for 8-10d while housed in separate cages in a 12:12h light-dark (L-D) cycle (lights on at 07:00h). Recordings of body temperature, heart rate, and spontaneous activity were made throughout. The temperature and heart rate rhythms were “purified” to take into account the effects of activity, and then the rhythm of temperature was further purified to take into account other masking influences (“non-activity masking effects” or NAME,). The methods employed in the purification processes involved linear regression analysis or analysis of covariance, the latter using functions of activity and NAME as covariates. From these methods, it was possible to obtain not only an estimate of the endogenous component of the temperature rhythm but also a measure of circadian changes in the sensitivity of temperature to masking effects.

Even though all purification methods removed many of the effects of spontaneous activity from the temperature record, there remained temperature fluctuations at the L-D and D-L transitions that appeared to be independent of activity. The NAME was of only very marginal value in the purification process. Comparison of the purification methods indicated that the linear methods were inferior (both from a biological viewpoint and when the results were compared mathematically) to those that allowed the rate of rise of temperature due to increasing amounts of activity to become progressively less. The sensitivity of temperature and heart rate to the masking effects of activity showed a circadian rhythm, with sensitivities in the resting phase being greater than those in the active phase. These findings are compatible with the view that thermoregulatory reflexes are induced by spontaneous activity of sufficient amount, and that there is a circadian rhythm in the body temperature at which these reflexes are initiated and in their effectiveness.     

Sleep and Circadian Rhythms of Temperature and Urinary Excretion on a 22.8 hr “Day”

Two groups of subjects (total N = 6) were studied in an isolation chamber for a period of 3 weeks whilst living on a 22.8 hr “day”. Regular samples of urine were taken when the subjects were awake, deep body temperature was recorded continuously and polygraphic EEG recordings were made of alternate sleeps. The excretion in the urine of potassium, sodium, phosphate, calcium and a metabolite of melatonin were estimated.

Measurements of the quantity and quality of sleep were made together with assessments of the temperature profiles associated with sleep. In addition, cosinor analysis of circadian rhythmicity in urinary variables and temperature was performed.

The 22.8 hr “days” affected variables and subjects differently. These differences were interpreted as indicating that the endogenous component of half the subjects adjusted to the 22.8 hr “days” but that, for the other three, adjustment did not occur. When the behaviour of different variables was considered then some (including urinary potassium and melatonin, sleep length and REM sleep) appeared to possess a larger endogenous component than others (for example, urinary sodium, phosphate and calcium), with rectal temperature behaving in an intermediate manner. In addition, a comparison between different rhythms in any subject enabled inferences to be drawn regarding any links (or lack of them) that might exist between the rhythms. In this respect also, there was a considerable range in the results and no links between any of the rhythms appeared to exist in the group of subjects as a whole.

Two further groups (total N=8) were treated similarly except that the chamber clock ran at the correct rate. In these subjects, circadian rhythms of urinary excretion and deep body temperature (sleep stages and urinary melatonin were not measured) gave no evidence for deterioration. We conclude, therefore, that the results on the 22.8 hr “day” were directly due to the abnormal “day” length rather than to a prolonged stay in the isolation chamber.     

Circadian Determinants of the Postlunch Dip in Performance

This study compared rectal temperature rhythms in groups of subjects who either did (n = 5) or did not (n - 7) show a clear postlunch dip in performance at a monotonous (25-30 min) vigilance task. Performance was tested every 2h in a standardized routine with lunch replaced by hourly liquid food supplements. Those showing the postlunch performance dip had a higher amplitude and later peaking 12h component to their rectal temperature rhythm that those who did not, resulting in flat, rather than rising, temperatures over the 10:00-15:00 time interval. The effect could not be explained by intergroup differences in prior sleep, morningness, or gender, although there was a trend (p = 0.09) for the “dip” group to be slightly younger (21.8y vs. 24.2y). The postlunch dip appears to be an endogenous phenomenon individually determined, but related to the strength of the (12h) harmonic of the circadian system.     

Body temperature rhythm of the tree shrew, Tupaia belangeri.

The circadian rhythm of body temperature of the tree shrew Tupaia belangeri was studied by telemetry. The amplitude of the daily (or circadian) variation was found to be much larger than that of most endotherms (amplitude approximately 5 degrees C) and the bimodal shape of the rhythm differed from the cosine waveform that characterizes the temperature rhythms of most other species. In free-running conditions, as well as in the entrained state, the temperature rhythm remained synchronized to the rhythm of locomotor activity.     

Temperature profiles, and the effect of sleep on them, in relation to morningness-eveningness in healthy female subjects

There were 15 healthy female subjects, differing in their position on the “morningness-eveningness” scale, studied for 7 consecutive days, first while living a sedentary lifestyle and sleeping between midnight and 08:00 and then while undergoing a “constant routine.” Rectal temperature was measured at regular intervals throughout this time, and the results were subjected to cosinor analysis both before and after “purification” for the effects of physical activity. Results showed that there was a phase difference in the circadian rhythm of core temperature that was associated with the morningness score, with calculations that “morning types” would be phased earlier than “evening types” by up to about 3h. This difference in phase (which was also statistically significant when the group was divided by a median split into a “morning group” and an “evening group”) could not be attributed to effects of waking activity and existed in spite of the subjects keeping the same sleep-wake schedule. Moreover, it persisted when the subjects' data had been purified and when the data were obtained from the constant routine. That is, there was an endogenous component to this difference in phase of the core temperature. The morning group also showed a greater fall of core temperature during sleep; this was assessed in two ways, the main one being a comparison of constant routine and nychthemeral data sets after correction for any effects of activity. Even though the morning group was sleeping at a later phase of their circadian temperature rhythm than was the evening group, neither group showed a fall of temperature due to sleep that varied with time elapsed since the temperature acrophase. It is concluded that another factor that differs between morning and evening types is responsible for this difference. (Chronobiology International, 18(2), 227-247, 2001)