SPOILER ALERT! ~ Sleep requires optimal body temperature. ~ We tend to fall asleep when body temperature declines. ~ As body temperature declines, brain starts cooling off. ~ Brain cooling corresponds to increased melatonin circulation. ~ NREM (deep sleep) occurs when body reaches its lowest temperature. ~ Wakefulness occurs when body temperature starts to rise again.
Sleep is a state that requires low body and brain temperature. Particularly, we tend to fall asleep when the temperature in our brain and body is in rapid decline, which is typically 2 h before sleep. By contrast, interruption of body cooling is associated with insomnia.
Thermoregulation Promotes Sleep
Indeed, in a natural environment, sleep onset coincides with a reduction in environmental temperature which happens after dark, while awakening occurs right before dawn when temperatures start to rise.
In terms of physiological changes, entry into sleep is facilitated by vasoconstriction (i.e., constricted/tightened blood vessels) which promotes body cooling, but awakening occurs when the body is warming up again.
Paradoxically, thermoregulatory circuits (i.e., brain circuits involved in temperature regulation) cause some parts of the body to warm up in preparation for sleep, which in turn triggers vasodilation. In this case, vasodilation subsequently promotes body cooling
Furthermore, transition from wakefulness to NREM sleep (i.e., deep sleep) is accompanied by cooling of the brain. However, it is important to note that cooling of the body and brain is only the consequence of vasodilation, not of sleep.
Warming and Cooling of the Body Affect Sleep
Interestingly, taking a hot bath before bed time decreases the time it takes to fall asleep and increases sleep depth. In addition, warming up for up to 4 h between 1 to 8 h before going to bed increases slow wave sleep (i.e, deepest stage of NREM sleep) during which dreams can occur and decreases REM sleep (i.e., the dreams stage).
However, sleep is initiated when body temperature declines and NREM is associated with further reductions in the temperature. Shedding light on this counter-intuitive relationship highlights the role of vasodilation in body cooling.
Indeed, vasodilation is typically observed 2 h before the start of the first sleep episode while the person is still awake. Then, as sleep approaches core temperature and hear rate drop and core temperature reaches its lowest level 2 h after sleep onset.
Importantly, prior to sleep onset as core temperature falls, alertness is decreased and melatonin circulation is increased. Subsequently, transition from wakefulness to NREM is followed by brain cooling, which coincides with reduced cerebral metabolism, conservation of energy and promotion of immune regulation and circadian coordination.
Conversely, interruption of sleep leads to disruption of this cooling process, and for example postponing sleep onset disturbs circadian temperature rhythm. Similarly, disruption of the process of vasodilation before sleep onset is sufficient to disrupt sleep.
Further support for this hypothesis comes from studies showing that sleep difficulties in the elderly is related to deficits in normal thermoregulation. In addition, slight changes in skin temperature (without altering core temperature) of only 0.4° C can help individuals fall asleep sooner and even encourage deeper sleep among those who suffer from bouts of insomnia during the night.
Harding, E. C., Franks, N. P., & Wisden, W. (2019). The Temperature Dependence of Sleep. Frontiers in neuroscience, 13, 336. https://doi.org/10.3389/fnins.2019.00336