Core body temperature must drop by approximately 1 to 3 degrees to initiate and maintain deep sleep, and must rise by about 1 to 3 degrees for waking up.
"in order to fall asleep and stay deeply asleep, your body temperature actually has to drop by about 1 to 3 degrees. And in order to wake up feeling refreshed and energized, your body temperature actually has to increase by about 1 to 3 degrees." (said at 0:43:56)
The speaker describes the well-established circadian rhythm of human core body temperature: core temperature drops prior to sleep onset, reaches its nadir during sleep, and rises in the morning to promote wakefulness. However, the claim needs qualification: the 1 to 3 degree magnitude corresponds to degrees Fahrenheit (approx. 0.5 to 1.5 °C); a drop of 1 to 3 degrees Celsius would represent substantial hypothermia. Furthermore, while rapid core cooling facilitates sleep onset and correlates with slow-wave (deep) sleep, phrasing the temperature change as an absolute mandatory requirement simplifies a complex, multi-factorial circadian and homeostatic process.
- supports: Nighttime drop in body temperature: a physiological trigger for sleep onset? (Sleep 1997)
"During disentrainment, there were significant correlations between the interval from MROD to sleep onset and the amount of slow-wave sleep (SWS) obtained during the sleep bout... It is suggested that a rapid decline in core body temperature increases the likelihood of sleep initiation and may facilitate an entry into the deeper stages of sleep." (abstract, results)
pubmedfull study (doi)