Keep the bedroom at 18–20°C for sleep

A cool sleeping environment provides the thermal gradient the body needs to drop core temperature and initiate deep sleep.

Why it works

Core body temperature falls 0.5–1.5°C during normal sleep initiation, driven by peripheral vasodilation that increases heat loss through the skin. This requires that the ambient temperature be lower than skin surface temperature so heat can radiate outward. In a warm room, the gradient narrows or reverses, impairing heat loss, prolonging sleep-onset latency, and reducing slow-wave sleep depth in the first half of the night.

How to do it

1. Set bedroom temperature to 18–20°C (65–68°F) before sleep; err cooler if you run warm or have heavy bedding.
2. Use a thermostat timer or set it 30 minutes before your target sleep time so the room is already at temperature.
3. If temperature control is limited, use a fan directed at your face and upper body — air movement increases convective heat loss even at the same absolute temperature.

Evidence

Multiple sleep studies find that ambient temperatures in the 18–20°C range are associated with shorter sleep-onset latency and greater slow-wave sleep compared to warmer environments; extreme cold or heat both impair sleep quality. (rct)

Optimal temperature varies considerably with age (older adults sleep better slightly warmer), sex, BMI, and bedding type; the 18–20°C guideline is a population central tendency, not a universal prescription.

Sources

• Muzet et al. (1984), ambient temperature and sleep in humans, Journal of Thermal Biology
• Okamoto-Mizuno & Mizuno (2012), effects of thermal environment on sleep, J. Physiological Anthropology

Common mistake

Bundling up in a warm room because the blanket feels comforting — the skin needs to radiate heat to the environment; heavy blankets in a warm room trap that heat and impair the core temperature drop.