Temperature and sleep
Throughout human evolution, temperature has been a strong cue for sleep and wake time. It serves as a so-called “zeitgeber” or time-giver, which is an external cue influencing our circadian rhythm. When researchers looked at sleep in three geographically distinct pre-industrial societies, they found that subjects fell asleep just as the external temperature began to drop and continued as temperatures declined. Similarly, waking up often occurred before sunrise when the ambient temperature was at its lowest and corresponded with the narrowing of blood vessels, or vasoconstriction, as indicated by the temperature of the fingers.

Additionally, thermoregulation and temperature impact the different stages of sleep. It’s been found that about two hours before sleep onset, your core body temperature begins to drop and reaches its lowest point during slow-wave sleep. Additionally, the temperature in the brain also slightly decreases at this time.

Our core body temperature hovers around 98.6 °F (37 °C) but fluctuates by about two degrees Fahrenheit during the night.

Decrease in one’s core body temperature during sleep stems from reducing our metabolic output, decreased muscle tone, and decreased sympathetic tone from vasodilation leading to heat loss distally.Valerie Cacho, MD, Integrative Sleep Physician, and Women’s sleep exper

According to Dr. Cacho, while melatonin is most often associated with light exposure, a cool room and the drop in body temperature can also help to increase the production of this “sleep hormone.”

In case the external temperature is too warm, studies have found it to have a negative impact on both the rapid eye movement (REM) phase of sleep and slow-wave sleep (SWS). Dreams occur in the REM sleep phase, when your brain is highly active. On the other hand, SWS is associated with less brain activity but increased physiological recoveries, such as the release of human growth hormone, muscle recovery, and the flushing of toxic material, including beta-amyloid plaques, from the brain. However, both are very important for optimal physical and cognitive well-being.

Looking at 10 elderly men’s responses to higher bedroom temperatures and their sleep quality, researchers found that even mild heat exposure during the nighttime sleep period would increase the thermal load, decrease REM, and increase wakefulness and whole-body sweat loss.

In addition, research at the University of California, Los Angeles found that warm-blooded animals with higher body temperatures have lower amounts of REM sleep, while those with lower body temperatures have more REM sleep.

Humidity also contributes to sleep quality. One study reported that humid heat exposure increased wakefulness and decreased REM sleep and SWS. Humidity also suppressed the decrease in core body temperature.

On the contrary, when your room is too cold, Dr. Kelvas claimed that the body may need to expend energy to stay warm, which can keep it from entering a deeper sleep.

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