CHANGES IN SLEEPINESS AND BODY-TEMPERATURE PRECEDE NOCTURNAL SLEEP ONSET - EVIDENCE FROM A POLYSOMNOGRAPHIC STUDY IN YOUNG MEN

Recent research has shown a close temporal relationship between the no cturnal decrease in rectal core temperature and the initiation of slee p. However, there is not yet a clear temporal relationship between cha nges in peripheral and core temperatures and nocturnal sleep onset. We recorded body temperatures in 14 adult males (age +/- SEM = 22.1 +/- 0.6 y), who attended the sleep laboratory for an adaptation night and two counterbalanced experimental sessions. Subjects self-selected ligh ts-out on one experimental night (the Habitual Sleep condition). To de termine the relationship between body temperature changes and sleep on set, lights out was delayed until after 01.00 hours on the other exper imental night (Delayed Sleep condition). Individual datasets in both c onditions were expressed relative to the time of sleep onset in the Ha bitual Sleep condition only, so that they were aligned at identical cl ock times. Saliva samples confirmed that mean dim light melatonin onse t (DLMO) occurred at 00.10 +/- 00.16 hours in the Delayed Sleep condit ion, which was after habitual sleep onset at 23.44 +/- 00.08 hours. Re ctal core temperature (Tc) decreased significantly over time only in t he Habitual Sleep condition (P < 0.01). For the 20 min before habitual sleep onset, Delayed Sleep Tc was on average 0.1 degrees C higher tha n Tc in the Habitual Sleep condition (P < 0.01). The greater decline i n Habitual Sleep Te was associated with significantly increased periph eral hand and foot skin temperatures before sleep (both P < 0.05). Sub jective sleepiness measures were higher in the Habitual Sleep onset co ndition from 150 min prior until sleep onset (P < 0.01). From these re sults it is reasonable to infer that a sequence of thermoregulatory an d sleep propensity changes occur before, but are associated with habit ual sleep onset, as the changes are significantly attenuated if sleep is delayed.