This study investigated the effect of diurnal thermal changes on the circadian rhythm of human core body temperature, together with secondary effects on other psycho-physiological states. Four male subjects were observed under two experimental temperature cycles in a climate chamber. In Case 1, the temperature remained fixed at 25°C (control); in Case 2, a constant temperature of 25°C during the morning was raised to 28°C at noon and remained constant during the afternoon and evening, to reflect the normal circadian increase in core body temperature (CBT) in the afternoon. The rectal temperature (RT) of each subject was monitored to evaluate the effect of the chamber temperature on the circadian rhythm of his core body temperature. Heart rate variability (HRV) and wrist activity were continuously monitored by electrocardiographs and wrist sensors. Subjective sleep quality was assessed just after awakening by a self-rating sleep questionnaire. For all subjects, Case 2 produced an afternoon RT significantly higher than Case 1. This and previous experiments indicate that controlling the thermal environment may increase the amplitude or advance the phase of the RT circadian rhythm, and can also affect the circadian rhythm of HRV. However, this experiment also indicated that changes in RT and HRV circadian rhythms due to varying the diurnal thermal environment may have little effect on the subjective quality of sleep.
