Best Temperature for Sleep: What Deep Sleep Research Shows

Ask what the best temperature for sleep is and you'll usually get a single number back. The research is less tidy than that. It gives you a range, several caveats and one finding that holds up well: a hot room is harder on deep sleep than a cool one.

Here's what the studies actually measured, who took part, and where the evidence runs out.

Why temperature and deep sleep are linked

Your core body temperature changes over the day, and that rhythm is closely tied to sleep. A review in Frontiers in Neuroscience by Harding, Franks and Wisden describes how core temperature starts falling about two hours before sleep onset. When people get to pick their own bedtime, they tend to choose the point where it's dropping fastest. The low point comes roughly two hours after lights out. To get rid of that heat, the body widens blood vessels in the hands and feet so warmth can escape through the skin.

Slow-wave sleep, the stage your wearable calls deep sleep, is concentrated in the first part of the night. That's exactly when your body is working hardest to lose heat. So it makes sense that a room that gets in the way of that heat loss would show up in deep sleep first.

What the lab studies found

Most of the direct evidence comes from sleep labs using polysomnography, the clinical standard that reads brain waves. A 2012 review in the Journal of Physiological Anthropology by Okamoto-Mizuno and Mizuno pulls the lab work together. The main points:

• Heat cuts deep sleep. With normal bedding and clothing, heat exposure increased time awake and reduced both slow-wave sleep and REM, in young adults and in older adults.
• Humidity makes it worse. In one study, 32°C (about 90°F) at 80% relative humidity early in the night reduced slow-wave sleep in that part of the night and pushed some of it later.
• People don't seem to adapt quickly. One study the review cites found heat still disrupted sleep after five nights of continuous exposure.
• Cool rooms are better tolerated. When people had their usual bedding, cold rooms didn't change sleep stages in the studies covered, which ran from about 3°C to 23°C. Without covers, cold mainly affected REM, not slow-wave sleep.

The review also mentions a thermoneutral point of 29°C (about 84°F). Don't take that as a bedroom target. It applies to people lying semi-nude with no covers. Blankets build a warm pocket of air around you, which is why a much cooler room feels comfortable.

One caveat: the review doesn't give sample sizes for the studies it covers, and lab studies like these tend to be small and short.

So what's the number?

Two figures come up in the research. The Harding review puts the best room temperature at roughly 19 to 21°C (66 to 70°F). It also says people try to keep the air right next to their skin, under the covers, between 31 and 35°C.

The largest real-world study I found is Baniassadi and colleagues (2023) in Science of the Total Environment. They followed 50 older adults in Boston, average age 79, with Oura rings and bedroom sensors, for 10,903 nights in total. Sleep was most efficient and restful when the bedroom was between 20 and 25°C (68 to 77°F). As the room warmed from 25 to 30°C, sleep efficiency dropped by 5 to 10%. Cooling from 22°C down to 14°C was linked to a drop of about 5%. Heat looked more costly than cold here too.

Before you lean on that study, note two things. It didn't measure deep sleep at all, only efficiency, total sleep time and restlessness. And people varied a lot: the link between temperature and sleep efficiency didn't even run in the same direction for everyone. The authors say themselves that the ideal setting is personal.

A fair reading: if you sleep under normal covers, the high 60s °F (around 19 to 21°C) is a sensible place to start. The case against a hot, humid room is stronger than any case for one exact number.

The warm bath paradox

This one sounds backwards. Warming yourself up before bed seems to help. A meta-analysis in Sleep Medicine Reviews by Haghayegh and colleagues (2019) found 17 studies that met its criteria and pooled data from 13. A bath or shower at 40 to 42.5°C (104 to 108.5°F), taken one to two hours before bed, was linked to better self-rated sleep quality and sleep efficiency. Even about ten minutes shortened the time it took to fall asleep. The abstract doesn't report slow-wave sleep, though the Harding review cites work where warming one to eight hours before bed increased it.

The proposed mechanism is the same one described above. Warm water sends blood to the hands and feet, and once you're out of the water your body sheds heat faster. Small lab work points the same way. In Raymann and Van Someren's 2008 study in SLEEP, with just eight people per group, gently warming the skin shortened sleep onset by about 18% in healthy older adults. That study only looked at falling asleep, not at deep sleep.

If you have a heart condition or tend to feel lightheaded in hot water, check with your doctor before making hot baths a habit.

What your wearable can and can't see

If you're judging your bedroom by your deep sleep numbers, it helps to know how good those numbers are. Robbins and colleagues (2024) in Sensors compared three devices with polysomnography in 35 healthy adults aged 20 to 50, over one night in a lab. On average, Oura's deep sleep estimate didn't differ significantly from polysomnography. The Apple Watch Series 8 came in 43 minutes low (51 minutes against 94), and Fitbit came in 15 minutes low. Night-by-night agreement was poor for all three devices. Oura funded the study, and one author sits on Oura's medical advisory board. Whoop wasn't tested.

What that means in practice: one night's deep sleep figure is a rough estimate. The more useful comparison is your own device against itself, across many nights, with only one thing changed.

How to test your own bedroom

Put a cheap thermometer by the bed, since the thermostat in the hallway may read quite differently. Pick one target, maybe 19°C for two weeks, and keep everything else the same: bedding, bedtime, alcohol, late meals. Watch humidity too, given what the lab studies found. Then compare against a normal stretch of nights, not against last Tuesday.

That's the kind of comparison Somnia Deep Sleep is built for. It runs a 14-night experiment on one habit, such as a warm room, and compares your deep sleep from Apple Health against your own 30-night baseline. Each result gets a label: Strong signal, Likely or Too early to tell. With wearable noise this large, Too early to tell is often the honest answer.

When it isn't the room

Temperature is worth adjusting, but it won't fix everything. See a doctor if you snore loudly with gasping or pauses in breathing, if you've had trouble sleeping most nights for more than three months, or if daytime sleepiness affects your driving. The same goes if you keep waking hot and sweaty however cool the room is.

What the research can't tell you

The evidence has real gaps. The lab studies that measured slow-wave sleep directly were mostly small and short, and done in conditions that aren't much like your bedroom. The biggest home study looked only at older adults in one city, used a wearable rather than polysomnography, didn't measure deep sleep, and was observational, so it shows association, not cause. None of the studies I read assigned people to different bedroom temperatures at home for weeks while measuring slow-wave sleep with polysomnography. Bedding, sleepwear, a partner's body heat and humidity all change what a given room temperature feels like, and most studies can't fully account for them. And individual differences were large wherever anyone looked.

So the answer to the best temperature for sleep is a starting range plus your own data, not one number.

Sources

• Harding EC, Franks NP, Wisden W. The Temperature Dependence of Sleep. Frontiers in Neuroscience. 2019;13:336. PMC6491889
• Okamoto-Mizuno K, Mizuno K. Effects of thermal environment on sleep and circadian rhythm. Journal of Physiological Anthropology. 2012;31:14. PMC3427038
• Baniassadi A, Manor B, Yu W, Travison T, Lipsitz L. Nighttime ambient temperature and sleep in community-dwelling older adults. Science of the Total Environment. 2023. PMC10529213
• Haghayegh S, Khoshnevis S, Smolensky MH, Diller KR, Castriotta RJ. Before-bedtime passive body heating by warm shower or bath to improve sleep: A systematic review and meta-analysis. Sleep Medicine Reviews. 2019;46:124-135. EM-Consulte abstract
• Raymann RJEM, Van Someren EJW. Diminished capability to recognize the optimal temperature for sleep initiation may contribute to poor sleep in elderly people. SLEEP. 2008;31(9):1301-1309. PMC2542970
• Robbins R, Weaver MD, Sullivan JP, et al. Accuracy of Three Commercial Wearable Devices for Sleep Tracking in Healthy Adults. Sensors. 2024;24(20):6532. PMC11511193

Want to know whether a cooler room actually changes your own deep sleep? Join the Somnia beta at /somnia/#join and run the experiment on your own nights.

This article is general information, not medical advice.