This project is designed to investigate possible thermoregulatory functions of sleep and sleep stages. There is considerable evidence that slow wave sleep, which is prevalent in the first part of sleep, is associated with a process that down-regulates body temperature. Consistent with a possible temperature-lowering function, slow-wave sleep can be augmented by heating prior to sleep. Furthermore, animals that exhibit torpor or hibernation enter these states through slow-wave sleep. Because many of the processes that occur during REM sleep produce heat in the central nervous system, we hypothesized that REM sleep has a thermogenic function associated with emergence from the shallow torpor of sleep (REM sleep is prevalent in the last part of sleep). Based on the hypothesis that REM sleep is a regulated form of CNS thermogenesis, we predicted that cooling the forehead during sleep would increase REM sleep, and that heating would decrease it. Ten normal individuals participated in an experiment in which their forehead was heated on one occasion and cooled on another occasion during an entire night's sleep, while sleep EEG and rectal, tympanic, eyelid, forehead and trunk skin temperatures were continuously monitored. Heating and cooling raised and lowered, respectively, forehead and eyelid temperatures, as expected. There was no consistent effect of heating or cooling on any of the sleep stages. In this experiment we also looked for possible effects on the frequency of sleep spindles. We examined sleep spindles, because the electrical activity designated by this label matches the description of electrical signals that are known to mediate effects of thermoregulatory controller systems on thermoeffector systems. Consistent with a possible counteregulatory response in a thermoregulatory system, the frequency of spindles was much lower in the cooling than in the warming condition. In light of the uniformly negative effects of the heating and cooling procedures on all other aspects of sleep physiology that were measured, the effects on sleep spindles are all the more striking. The lack of effects on slow wave sleep and REM sleep do not rule out a thermoregulatory role for these sleep stages. The stimulus applied in this experiment may be too weak to elicit a response, or it may be irrelevant to their mode of action.
