The circadian rhythm of temperature in humans during a 26-hr sleep-wake schedule

The effect of non-24-hr zeitgebers on human circadian rhythms is usually studied in temporal isolation units. In this study, subjects tried to adhere to a 26-hr sleep-wake schedule while living at home exposed to the conflicting natural 24-hr zeitgebers. Temperature was continuously measured with a rectal probe. Daily sleep logs provided subjective estimates of sleep and wake times. After a baseline period on a 24-hr schedule, the subjects followed the 26-hr schedule for 12-13 consecutive days. The circadian rhythm of temperature appeared entrained to the 26-hr schedule for two of the subjects (the adapters), but was definitely not entrained for the other two subjects (the non-adapters). For the non-adapters a 24-hr component persisted in the temperature rhythm. Sleep time conformed more closely to the planned schedule for the adapters than for the non-adapters. These results show that a 26-hr sleep-wake schedule can be a more powerful zeitgeber than the natural 24-hr zeitgebers. Factors that might determine whether an individual will entrain to the 26-hr schedule are discussed.