If 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.
When researchers looked at 10 elderly men’s responses to higher bedroom temperatures and their sleep quality, they 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.
Additionally, 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 is also a contributing factor 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 explained that the body may need to expend energy to stay warm, which can keep it from entering a deeper sleep.
