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)     

The Adjustment of the Circadian Rhythm of Body Temperature to Simulated Time-Zone Transitions: A Comparison of the Effect of Using Raw Versus Unmasked Data

Deep body temperature and sleep/activity diaries data were recorded during control days and for 6 days after simulated time zone transitions of 8 h to the east (six subjects) or west (seven subjects). Circadian rhythms were assessed by cosinor analysis of both raw data (the conventional method) and purified data (corrected for the effects of sleep and activity). Analysis of raw data gives misleading information about the phase and amplitude of the rhythms due to the masking effects of the exogenous component. Use of purified data indicates that during the process of adjustment after an eastward shift (a) phase changes are more erratic than after a shift to the west; (b) no marked decrease in the amplitude of the rhythms is evident; and (c) no clear evidence exists that the circadian rhythm breaks up temporarily. The masking effect was less after the time zone transition if sleep maintenance was poor.     

The Adjustment of the Circadian Rhythm of Body Temperature to Simulated Time-Zone Transitions: A Comparison of the Effect of Using Raw Versus Unmasked Data

Deep body temperature and sleep/activity diaries data were recorded during control days and for 6 days after simulated time zone transitions of 8 h to the east (six subjects) or west (seven subjects). Circadian rhythms were assessed by cosinor analysis of both raw data (the conventional method) and purified data (corrected for the effects of sleep and activity). Analysis of raw data gives misleading information about the phase and amplitude of the rhythms due to the masking effects of the exogenous component. Use of purified data indicates that during the process of adjustment after an eastward shift (a) phase changes are more erratic than after a shift to the west; (b) no marked decrease in the amplitude of the rhythms is evident; and (c) no clear evidence exists that the circadian rhythm breaks up temporarily. The masking effect was less after the time zone transition if sleep maintenance was poor.     

Estimates of the Daily Phase and Amplitude of the Endogenous Component of the Circadian Rhythm of Core Temperature in Sedentary Humans Living Nychthemerally

Fifteen healthy female subjects were studied for eight days while living conventionally. Subjects were free to choose the ways they spent their time within a framework of regular times of retiring and rising; in practice, much of the waking time was spent in sedentary activities. Nine of the subjects were aware of the natural light-dark cycle, this approximating to a 12:12 L:D schedule at the time of year when the study took place. Before the study, subjects were assessed for their degree of "morningness" by questionnaire; throughout the study, they wore a rectal probe, and an activity meter on their non-dominant wrist. The timing (phase) and amplitude of the circadian rectal temperature rhythm were assessed on each day by cosinor analysis as well as by a me thod based on visual inspection of the data. These two parameters were also assessed after the temperature data for each day had been "purified" by a number of methods. From these results it was possible to investigate the effect of purification upon the amplitude of the circadian rhythm of temperature. Also, the day-by-day variability of phase, and the relationship between morningness and phase, were compared using these methods of phase estimation, and using cross-correlation between data sets from adjacent days; in all cases, raw and purified temperature data were used. There was a significantly greater amount of daily variation in phase using purified rather than raw data sets, and this difference was present with all methods of purification as well as with all methods for estimating phase. Purifi cation decreased the amplitude of the circadian temperature rhythm by about 30%. Finally, there was a significant correlation between the morningness score of the subjects and the phase of the circadian temperature rhythm, the phase becoming earlier with increasing morningness; when this relationship was re-examined using purified data, it became more marked. These results reflect the masking effects exerted upon raw temperature data by lifestyle. The extent to which the purification methods enable the endogenous component of a circadian rhythm - and, by implication, the output of the endogenous circadian oscillator - to be estimated in subjects living normally is addressed.     

Estimates of the Daily Phase and Amplitude of the Endogenous Component of the Circadian Rhythm of Core Temperature in Sedentary Humans Living Nychthemerally

Fifteen healthy female subjects were studied for eight days while living conventionally. Subjects were free to choose the ways they spent their time within a framework of regular times of retiring and rising; in practice, much of the waking time was spent in sedentary activities. Nine of the subjects were aware of the natural light-dark cycle, this approximating to a 12:12 L:D schedule at the time of year when the study took place. Before the study, subjects were assessed for their degree of "morningness" by questionnaire; throughout the study, they wore a rectal probe, and an activity meter on their non-dominant wrist. The timing (phase) and amplitude of the circadian rectal temperature rhythm were assessed on each day by cosinor analysis as well as by a me thod based on visual inspection of the data. These two parameters were also assessed after the temperature data for each day had been "purified" by a number of methods. From these results it was possible to investigate the effect of purification upon the amplitude of the circadian rhythm of temperature. Also, the day-by-day variability of phase, and the relationship between morningness and phase, were compared using these methods of phase estimation, and using cross-correlation between data sets from adjacent days; in all cases, raw and purified temperature data were used. There was a significantly greater amount of daily variation in phase using purified rather than raw data sets, and this difference was present with all methods of purification as well as with all methods for estimating phase. Purifi cation decreased the amplitude of the circadian temperature rhythm by about 30%. Finally, there was a significant correlation between the morningness score of the subjects and the phase of the circadian temperature rhythm, the phase becoming earlier with increasing morningness; when this relationship was re-examined using purified data, it became more marked. These results reflect the masking effects exerted upon raw temperature data by lifestyle. The extent to which the purification methods enable the endogenous component of a circadian rhythm – and, by implication, the output of the endogenous circadian oscillator – to be estimated in subjects living normally is addressed.     

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)     

Can Insulated Skin Temperature Act as a Substitute for Rectal Temperature when Studying Circadian Rhythms?

We measured rectal, lateral chest wall, and axillary temperature every half hour for at least 24 h while subjects were living normal life-styles and keeping a sleep/activity diary. We then used a purification method to estimate the decrease of temperature due to sleep and the increases due to sitting, standing, walking, or exercising, as well as the parameters of the cosine curve that described the “purified data.” Cosinor analysis of raw and purified data showed that the acrophases from both skin sites were much more variable and up to 8 h later than were those from the rectum (particularly if exercise had been taken), even though the acrophases from the two skin sites were similar to each other. For rectal temperature, there was an increase in the size of the masking effect as activity progressed through the sequence: sitting, standing or walking, exercising. In contrast, for both chest wall and axillary temperatures, although sitting produced masking effects similar to those for rectal temperature, masking effects due to standing or walking and exercising were much smaller, and sometimes they were even less than the masking effects due to sitting. These results indicate that our measurements of cutaneous temperature did not act as a substitute for rectal temperature, particularly when the subject was physically active rather than sedentary.     

Measuring phase shifts in humans following a simulated time-zone transition: agreement between constant routine and purification methods

Twelve healthy participants were studied in an Isolation Unit. For the first 7 (control) days, subjects lived on UK time. Then the clock was advanced by 8 h, mimicking an eastward time-zone transition, and for days 8 to 12, participants lived on this new local time. Two constant routines (participants were not allowed to sleep, were restricted in movement, and ate regular, identical snacks) were undertaken, during the control days (days 3 to 4) and at the end of the experiment (days 11 to 12). Rectal temperature and activity were measured throughout, with activity used to correct the measured temperatures for the direct (masking) effects of the sleep-wake cycle. Phase changes of the temperature rhythm between the constant routines were assessed by cross-correlation and cosinor analysis. During days 8 to 10, the measured temperatures and those that had been corrected (purified) for masking were assessed by the same two methods, and the shifts were extrapolated to predict the values expected during the second constant routine. Individuals differed widely in the phase shifts of the temperature rhythm, but the correlations between the changes measured by constant routines and those estimated by the purification methods were high (r=0.771 to 0.903), and the differences between them were not significantly different from zero (p>0.24). Phase shifts of the measured (masked) temperature rhythm were poorer predictors of the shift obtained from the constant routines (r3+/-4.5 h). Limitations of the methods due to the variability of results are discussed, but we conclude that the mean phase shifts obtained from purified, but not raw, temperature data show acceptable agreement with those found using our version of the constant routine.     

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.     

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.     

"Demasking" the temperature rhythm after simulated time zone transitions.

Simulated time zone transitions were performed in an isolation unit upon groups of one to four human subjects. In the first series of experiments, the adjustment of the circadian rhythm of body temperature, measured in the presence of sleep and other masking factors, was assessed by cosinor analysis and by cross-correlation methods. These methods modeled the circadian timing system either as a single component or as the sum of two components, those due to exogenous and endogenous influences. The one-component models described a more rapid adjustment of the temperature rhythm to the time zone transition than did the two-component models; we attribute this difference to the masking effects of the exogenous component. In a second series of experiments, we showed that the shift of the endogenous component, as assessed by the two-component models, was not significantly different from that measured during constant routines. The results also showed that, if the zeitgebers were phased in advance of the endogenous component, then advances of the endogenous component were produced only if this mismatch was less than about 10 hr. Mismatches greater than this, and cases where the zeitgebers were delayed with respect to the endogenous component, both produced delays of the endogenous component. We conclude that the two-component cross-correlation methods can be used to estimate shifts of the endogenous component of a circadian rhythm in the presence of masking factors. They are therefore an alternative to constant routines when these latter are impracticable to carry out.     

Measuring Phase Shifts in Humans Following a Simulated Time‐Zone Transition: Agreement Between Constant Routine and Purification Methods

Twelve healthy participants were studied in an Isolation Unit. For the first 7 (control) days, subjects lived on UK time. Then the clock was advanced by 8 h, mimicking an eastward time‐zone transition, and for days 8 to 12, participants lived on this new local time. Two constant routines (participants were not allowed to sleep, were restricted in movement, and ate regular, identical snacks) were undertaken, during the control days (days 3 to 4) and at the end of the experiment (days 11 to 12). Rectal temperature and activity were measured throughout, with activity used to correct the measured temperatures for the direct (masking) effects of the sleep‐wake cycle. Phase changes of the temperature rhythm between the constant routines were assessed by cross‐correlation and cosinor analysis. During days 8 to 10, the measured temperatures and those that had been corrected (purified) for masking were assessed by the same two methods, and the shifts were extrapolated to predict the values expected during the second constant routine. Individuals differed widely in the phase shifts of the temperature rhythm, but the correlations between the changes measured by constant routines and those estimated by the purification methods were high (r=0.771 to 0.903), and the differences between them were not significantly different from zero (p>0.24). Phase shifts of the measured (masked) temperature rhythm were poorer predictors of the shift obtained from the constant routines (r≤0.605; mean±SD of differences >3±4.5 h). Limitations of the methods due to the variability of results are discussed, but we conclude that the mean phase shifts obtained from purified, but not raw, temperature data show acceptable agreement with those found using our version of the constant routine.     

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)     

Circadian Temperature and Activity Rhythms in Mice under Free-Running and Entrained Conditions; Assessment after Purification of the Temperature Rhythm

Six female mice were studied separately for six weeks, first in constant light (300 lx), and then on a 12 : 12 L : D schedule (light on 07:00–19:00–h). Food and water were available ad libitum. Abdominal temperature and spontaneous locomotor activity were measured every 10 min. In constant light, the animals free-ran with both temperature and activity records showing circadian rhythms that were significantly greater than 24 h; by contrast, in the LD schedule, the circadian rhythms had become entrained and showed a stable phase relation to this schedule. The direct masking effects upon raw temperatures caused by bursts of activity were clearly seen, and could be removed by a process of ‘purification’. A comparison of the activity profiles during the entrained and free-running phases showed that the imposed light-dark cycle resulted in decreased activity in the light, increased activity in the dark, and bursts of activity at the light-dark and dark-light transitions. Masking effects due to the activity profile were present in the raw temperature profile, and many could be removed by purification using the activity profile; however, there was evidence that other masking effects, independent of activity, were present also. The efficacy of thermoregulatory compensation, as assessed from the rise of core temperature produced by spontaneous locomotor activity, was, in comparison with the free-running condition, increased in the dark phase and decreased in the light phase; this would appear to be one way to limit the temperature rise that occurs in the active phase of the rest-activity cycle.     

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.     

Twenty-four-hour and Ultradian Rhythmicities in Healthy Full-Term Neonates: Exogenous and Endogenous Influences

Eleven healthy, full-term babies were studied on the second day after birth and again 4 weeks later. The babies lived on a 24-h light/dark cycle (light from 0700D1900) and were bottle-fed every 4 h. Systolic blood pressure, heart rate, skin (abdomen) and rectal temperatures were measured at 10-min intervals for 24 h on each occasion of study. The behavioural state of the baby was measured at the same time, and this information was used to purify the raw data (i.e., to separate it into the endogenous, clock-driven and exogenous, lifestyle-driven components). Raw and purified data were assessed for 24-h and ultradian (12-, 8-, 6-, 4-, 3-, 2-h) periodicities by cosinor analysis. We confirm the development of 24-h rhythmicity in skin and rectal temperatures between day 2 and week 4; at the same time, ultradian rhythms (4-h) developed in all variables. For heart rate and systolic blood pressure the development of a 4-h ultradian rhythm was in phase with the behavioural changes produced by feeding; by contrast, for the temperatures, these weak exogenous components were accompanied by a stronger 4-h component, that was out of phase with feeding. Masking effects due to sleep and activity changed in size between day 2 and week 4. Also, those positive masks produced by waking activities were more marked in the light, whereas the negative masks produced by sleep were more marked in the dark. Some implications of these results for the development of rhythmicity in infants, particularly whether due to lifestyle or the development of internal processes, are discussed.     

Circadian Temperature and Activity Rhythms in Mice under Free-Running and Entrained Conditions; Assessment after Purification of the Temperature Rhythm

Six female mice were studied separately for six weeks, first in constant light (300 lx), and then on a 12 : 12 L : D schedule (light on 07:00-19:00-h). Food and water were available ad libitum. Abdominal temperature and spontaneous locomotor activity were measured every 10 min. In constant light, the animals free-ran with both temperature and activity records showing circadian rhythms that were significantly greater than 24 h; by contrast, in the LD schedule, the circadian rhythms had become entrained and showed a stable phase relation to this schedule. The direct masking effects upon raw temperatures caused by bursts of activity were clearly seen, and could be removed by a process of 'purification'. A comparison of the activity profiles during the entrained and free-running phases showed that the imposed light-dark cycle resulted in decreased activity in the light, increased activity in the dark, and bursts of activity at the light-dark and dark-light transitions. Masking effects due to the activity profile were present in the raw temperature profile, and many could be removed by purification using the activity profile; however, there was evidence that other masking effects, independent of activity, were present also. The efficacy of thermoregulatory compensation, as assessed from the rise of core temperature produced by spontaneous locomotor activity, was, in comparison with the free-running condition, increased in the dark phase and decreased in the light phase; this would appear to be one way to limit the temperature rise that occurs in the active phase of the rest-activity cycle.     

The Circadian Rhythm of Core Temperature: Origin and some Implications for Exercise Performance

This review first examines reliable and convenient ways of measuring core temperature for studying the circadian rhythm, concluding that measurements of rectal and gut temperature fulfil these requirements, but that insulated axilla temperature does not. The origin of the circadian rhythm of core temperature is mainly due to circadian changes in the rate of loss of heat through the extremities, mediated by vasodilatation of the cutaneous vasculature. Difficulties arise when the rhythm of core temperature is used as a marker of the body clock, since it is also affected by the sleep‐wake cycle. This masking effect can be overcome directly by constant routines and indirectly by “purification” methods, several of which are described. Evidence supports the value of purification methods to act as a substitute when constant routines cannot be performed. Since many of the mechanisms that rise to the circadian rhythm of core temperature are the same as those that occur during thermoregulation in exercise, there is an interaction between the two. This interaction is manifest in the initial response to spontaneous activity and to mild exercise, body temperature rising more quickly and thermoregulatory reflexes being recruited less quickly around the trough and rising phase of the resting temperature rhythm, in comparison with the peak and falling phase. There are also implications for athletes, who need to exercise maximally and with minimal risk of muscle injury or heat exhaustion in a variety of ambient temperatures and at different times of the day. Understanding the circadian rhythm of core temperature may reduce potential hazards due to the time of day when exercise is performed.     

Can Insulated Skin Temperature Act as a Substitute for Rectal Temperature when Studying Circadian Rhythms?

We measured rectal, lateral chest wall, and axillary temperature every half hour for at least 24 h while subjects were living normal life-styles and keeping a sleep/activity diary. We then used a purification method to estimate the decrease of temperature due to sleep and the increases due to sitting, standing, walking, or exercising, as well as the parameters of the cosine curve that described the “purified data.” Cosinor analysis of raw and purified data showed that the acrophases from both skin sites were much more variable and up to 8 h later than were those from the rectum (particularly if exercise had been taken), even though the acrophases from the two skin sites were similar to each other. For rectal temperature, there was an increase in the size of the masking effect as activity progressed through the sequence: sitting, standing or walking, exercising. In contrast, for both chest wall and axillary temperatures, although sitting produced masking effects similar to those for rectal temperature, masking effects due to standing or walking and exercising were much smaller, and sometimes they were even less than the masking effects due to sitting. These results indicate that our measurements of cutaneous temperature did not act as a substitute for rectal temperature, particularly when the subject was physically active rather than sedentary.     

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.