Why a warm room costs you deep sleep

In this story

• Why it matters
• What heat does
• It replaces deep sleep with time awake
• You may not get used to it
• Older adults may feel it more
• Why a cool room usually wins
• Try this tonight
• Sources

Most people treat bedroom temperature as a comfort setting. The research treats it as something closer to a condition for sleep. Your body has to lose heat to fall asleep and stay asleep, and a warm room makes that harder.

Why it matters

A review of the research on temperature and sleep found the same pattern across studies: heat exposure increases wakefulness and decreases both slow-wave sleep and REM sleep.^[1]

That matters because those are not interchangeable hours. Most deep sleep happens early in the night, and that is also when the effects of heat were strongest.

What heat does

It replaces deep sleep with time awake

The review describes wakefulness as the stage that copes with a heat load. In other words, when the room runs hot, the night tends to trade deep and REM sleep for time awake.^[1]

You may not get used to it

It is tempting to assume the body adapts. One study cited in the review found that sleep disruption from heat did not settle down after five days of continuous exposure.^[1]

Older adults may feel it more

Even mild heat increased wakefulness and reduced REM sleep in older adults, according to the same review.^[1]

Why a cool room usually wins

Blankets explain a lot of this. They hold a warm pocket of air around your body, so you can add warmth in a cool room. It is much harder to get rid of warmth in a hot one.

Cool does not mean cold, though. The same review's authors advise keeping winter indoor temperatures above 10°C, and the Sleep Foundation says most doctors recommend a bedroom of 65 to 68°F.^[2]

Try this tonight

• Set the room to 65 to 68°F, and adjust from there for your own comfort.
• Use lighter covers you can add to or kick off, rather than one heavy layer.
• Cool the room before bed, so it is ready when you are.

This article is general education, not medical advice. If you are worried about your sleep, talk to a doctor.

Sources

1. 1.Okamoto-Mizuno K, Mizuno K (2012). Effects of thermal environment on sleep and circadian rhythm. Journal of Physiological Anthropology, 31(1):14. Accessed Nov 5, 2026.
2. 2.Pacheco D, Sleep Foundation (updated 2025). The Best Temperature for Sleep. Sleep Foundation. Accessed Nov 5, 2026.