Temperature

What It Is

The temperature of the air where you sleep.

Why It Matters

A cool bedroom supports sleep, and a warm one works against it. A systematic review of real-world studies found that higher indoor and outdoor temperatures are generally linked to shorter, more broken sleep, with the effect strongest on the hottest nights and in more vulnerable people. The direction of that finding is consistent; the exact best temperature is far less settled.

The reason is thermoregulation. Falling and staying asleep depends on your core body temperature dropping slightly at night, and a warm room slows that heat loss, increasing wakefulness and trimming deep and REM sleep. Cold matters less once you have bedding to offset it, so in real life heat is usually the bigger disruptor. In one study of older adults, sleep was most efficient between roughly 20 and 25°C and fell off as the room warmed toward 30°C, though the best point varied widely from person to person, which points to a comfortable band rather than the coldest possible room.

Keep the bedroom cool and well ventilated, and avoid going to bed overheated. There is no single number that suits everyone, so treat cool but comfortable as the goal rather than chasing a specific degree. Note that specialized cooling mattresses and bedding have not reliably improved sleep in controlled trials, so focus on the room itself and on removing excess heat rather than on special gear.

• Okamoto-Mizuno, K., Mizuno, K. (2012). Effects of thermal environment on sleep and circadian rhythm. Journal of Physiological Anthropology. doi.org/10.1186/1880-6805-31-14
• Baniassadi, A. et al. (2023). Nighttime ambient temperature and sleep in community-dwelling older adults. Science of the Total Environment. doi.org/10.1016/j.scitotenv.2023.165623
• Chevance, G. et al. (2024). A systematic review of ambient heat and sleep in a warming climate. Sleep Medicine Reviews. doi.org/10.1016/j.smrv.2024.101915
• Pasquier, F. et al. (2024). Does body cooling facilitated by bedding compared to control condition improve sleep among adults (18-64 years old)? A systematic review and meta-analysis. Journal of Thermal Biology. doi.org/10.1016/j.jtherbio.2024.104030