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-drive) 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 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.