Finding the perfect temperature for sleep has become an area of interest and research and for good reasons! As much as we focus on blue light and bright light, our sleep-wake cycle is also intimately tied to temperature. In fact, one of the leading theories for why we sleep has been for temperature regulation.
There are several biological rhythms that are tied to temperature including the ultradian menstrual cycle.
I like to think of the body as having two thermophysiological compartments, the core (organs) and the shell (muscles/skin/extremities). What’s really happening when our core body temperature lowers for sleep is that our blood volume is shifting from the shell to the core. When it rises in the day, it is shifting from core to shell.
In the natural world, the temperature outside is significantly colder at night, so shifting the blood to the core helps protect the body from heat loss. When this happens the peripheral circulation releases heat through the skin, concentrated in special capillary beds on the soles of the feet, the palms, and around the mouth.
So how does this apply to taking a hot bath or shower for sleep? A minimum of 10-minutes of hot water (has to be warmer than core body temperature) serves as water based passive body heating facilitating the loss of heat in preparation for the the thermophysiological changes. It is thought to help most 1-2 hours before bedtime and less so right at bedtime when the shift has already occurred.
Baths and showers can also be a behavioral cue for bedtime and serve as a wind down ritual that has dual value.
We also have a dip in core body temperature in the afternoon that corresponds with an increase in sleepiness that many people misappropriate to lunch.
Do you take a bath or shower before bedtime?