Best Bedroom Temperature for Sleep

Updated: 2026-09-25

"Sleep at 18°C" is a commonly repeated piece of sleep advice, but it doesn't trace back to one precise source. What does exist is a set of controlled studies that measured heat and cold in the bedroom in different ways. A 2012 review (Okamoto-Mizuno & Mizuno) found that both heat and cold exposure increase wakefulness and reduce REM and slow-wave sleep, through different mechanisms. A more recent observational study of older adults (Baniassadi et al., 2023) found sleep was most efficient between 20–25°C, with a clinically meaningful 5–10% drop in sleep efficiency as temperature rose from 25°C to 30°C — and large between-person variation. Below: what different studies actually measured, and how to check your own range against your bedroom sensor.

Why temperature affects sleep at all

A 2012 review by Okamoto-Mizuno and Mizuno describes the mechanism: overheating or overcooling both raise wakefulness and cut into REM and slow-wave (deep) sleep. In real-world conditions, with bedding and clothing in use, it's heat exposure specifically that increases wakefulness and reduces deep and REM sleep.

Cold is more nuanced: with bedding in use, cold exposure barely changed sleep stage structure in this review, but it did affect cardiac autonomic response during sleep — meaning the body reacts to cold even when sleep stages look normal.

What a controlled study in older adults found

A 2022 experimental study (Yan et al.) compared sleep in 16 people over 65 at 27°C versus 30°C with full polysomnography. At 30°C, total sleep time fell by 26.3 minutes, sleep efficiency dropped by 5.5%, REM sleep shrank by 5.3 minutes, and time awake rose by 27.0 minutes, compared with 27°C.

At the same temperature, improved ventilation added a further 10.3 minutes of deep sleep and 3.7 minutes of REM sleep.

The range that came out most efficient

An observational study by Baniassadi et al. (2023) tracked sleep and bedroom temperature in community-dwelling older adults using wearable sleep monitors and environmental sensors. Sleep was most efficient and restful between 20–25°C, and efficiency dropped by a clinically meaningful 5–10% as temperature rose from 25°C to 30°C.

The authors specifically flagged that the relationship was nonlinear, with "substantial between-subject variations" — meaning 20–25°C is a range for the sample as a whole, not a guaranteed personal optimum for any one individual.

Cold isn't free either

A survey of Japanese adults (Chimed-Ochir et al., 2021) found that the more often someone felt cold in their bedroom, the worse their PSQI sleep-quality score was, in a clear dose-response pattern: "sometimes cold" corresponded to +0.57 points, "always cold" to +2.25 points, compared with people who didn't feel cold at all.

There's also a counterexample that complicates the picture: a field study in Beijing (Zhang et al., 2021) with 104 participants found the neutral pre-sleep temperature was around 23.8°C and 26.5°C during sleep, and a warmer (not colder) pre-sleep sensation correlated with a higher deep-sleep percentage in that sample.

How to check this against your own data

Health OS can read a Xiaomi/Mijia temperature and humidity sensor next to your bed and line it up against your ring's sleep stages — so you can see your own deep-sleep-vs- bedroom-temperature relationship instead of relying on an average from someone else's study sample.

The "Sleeping in an 18°C Room" protocol runs 7 baseline days and 7 days at 18°C (with a warm blanket, cool air), comparing deep-sleep percentage and overnight HRV. Its built-in guardrails: avoid getting too cold, and return to a comfortable temperature if you catch a cold, feel chills, or can't fall asleep from the cold for 2+ nights in a row.

FAQ

What's the best temperature for sleep — is it 18°C?

There's no single proven number. An observational study of older adults (Baniassadi 2023) found the most efficient sleep between 20–25°C, with efficiency dropping as temperature rose toward 30°C — but with wide between-person variation. 18°C in Health OS's protocol is a testable hypothesis, not an established optimum.

Does hot weather really make sleep worse?

In older adults, going from 27°C to 30°C cut total sleep time by 26 minutes and sleep efficiency by 5.5% in a controlled experiment (Yan 2022).

Is a cold bedroom in winter also bad?

Feeling cold in the bedroom in winter correlated with a worse (higher) PSQI sleep-quality score, in a dose-dependent way (Chimed-Ochir 2021). So "just open the window" without a warm blanket isn't a universal fix.

Do I need air conditioning in summer?

Studies didn't test AC specifically, but they show that lowering heat and stuffiness improves sleep: in the older-adult experiment, better ventilation at the same temperature added 10.3 minutes of deep sleep.

How do I find my own ideal temperature?

By using a bedroom temperature sensor and comparing it against your own deep-sleep share night by night — that's what Health OS's Xiaomi/Mijia sensor integration and 7+7-day protocol are built for.