Cool the bedroom to 18–20°C (65–68°F)

Core body temperature must drop by about 1°C to initiate and maintain sleep — a cool room is not a preference, it’s a mechanism.

Why it works

Sleep onset coincides with a core body temperature drop driven by peripheral vasodilation — heat radiates from the hands and feet to the environment. A warm room impedes this heat dissipation, prolonging the time to reach the target temperature and delaying sleep onset. Core temperature also naturally rises in the second half of the night, coinciding with waking; an overheated room accelerates this rise and fragments late sleep.

How to do it

1. Set the bedroom thermostat to 18–20°C (65–68°F) for sleep, or lower if you run warm.
2. If you cannot control room temperature, use lighter bedding, a fan, or a cooling mattress pad.
3. A warm bath or shower 60–90 minutes before bed paradoxically helps — it briefly raises peripheral temperature, accelerating the rebound drop that signals sleep readiness.

Evidence

The relationship between core body temperature decline and sleep onset is well established in thermoregulatory sleep research; the optimal bedroom temperature range of 18–20°C is supported by multiple studies on sleep quality and nocturnal awakenings. The distal-warming mechanism is directly evidenced: Kräuchi et al. found that warm feet (peripheral vasodilation shedding core heat) predict rapid sleep onset, and a meta-analysis by Haghayegh et al. confirmed that a warm bath or shower 60–90 minutes before bed accelerates the core-temperature drop and shortens time to fall asleep. (rct)

Optimal temperature varies with individual physiology, bedding, and ambient humidity; the 18–20°C range is a population average, not a universal prescription.

Sources

• Lack et al. (2008), the relationship between insomnia and body temperatures, Sleep Medicine Reviews
• Lack LC, Gradisar M, Van Someren EJW, Wright HR, Lushington K (2008). The relationship between insomnia and body temperatures. Sleep Medicine Reviews, 12(4), 307–317.
• Kräuchi K, Cajochen C, Werth E, Wirz-Justice A (1999). Warm feet promote the rapid onset of sleep. Nature, 401(6748), 36–37.
• Haghayegh S, Khoshnevis S, Smolensky MH, Diller KR, Castriotta RJ (2019). Before-bedtime passive body heating by warm shower or bath to improve sleep: A systematic review and meta-analysis. Sleep Medicine Reviews, 46, 124–135.

Common mistake

Assuming body heat from exercise just before bed is fine because "you’re tired" — vigorous late exercise raises core temperature for 60–90 minutes, delaying the temperature drop that initiates sleep.