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 co nventionally. Subjects were free to choose the ways they spent their time w ithin a framework of regular times of retiring and rising; in practice, muc h of the waking time was spent in sedentary activities. Nine of the subject s were aware of the natural light-dark cycle, this approximating to a 12: 1 2 L:D schedule at the time of year when the study took place. Before the st udy, subjects were assessed for their degree of "morningness" by questionna ire; throughout the study, they wore a rectal probe, and an activity meter on their nondominant wrist. The timing (phase) and amplitude of the circadi an rectal temperature rhythm were assessed on each day by cosinor analysis as well as by a method based on visual inspection of the data. These two pa rameters were also assessed after the temperature data for each day had bee n "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 r elationship between morningness and phase, were compared using these method s of phase estimation, and using cross-correlation between data sets from a djacent days; in all cases, raw and purified temperature data were used. Th ere was a significantly greater amount of daily variation in phase using pu rified rather than raw data sets, and this difference was present with all methods of purification as well as with all methods for estimating phase. P urification decreased the amplitude of the circadian temperature rhythm by about 30%. Finally, there was a significant correlation between the morning ness score of the subjects and the phase of the circadian temperature rhyth m, the phase becoming earlier with increasing morningness; when this relati onship was re-examined using purified data, it became more marked. These re sults reflect the masking effects exerted upon raw temperature data by life style. The extent to which the purification methods enable the endogenous c omponent of a circadian rhythm - and, by implication, the output of the end ogenous circadian oscillator - to be estimated in subjects living normally is addressed.