Does a cool, dark bedroom really help you sleep better?

A cool bedroom does indeed help you sleep better, but colder is not always better. The most favourable temperature lies between 20 and 25 degrees, especially for older adults. A Japanese observational study¹ found that people who felt cold in their bedroom scored an average of 2.25 points worse on a sleep questionnaire. So too cold does not work either. On the other hand, your sleep efficiency drops measurably as soon as the room exceeds 25 degrees, as shown by a study using wearable sleep trackers in real homes².

How badly heat disrupts your sleep is clearly visible in an experiment with sixteen older adults³. At 30 degrees they slept an average of 26 minutes less than at 27 degrees, lay awake 27 minutes longer, and lost almost five minutes of REM sleep. Their nervous system was also measurably overactive. These are objective brain measurements, not self-reports. The group was small, but the outcomes align well with those of the other studies.

Ventilation is at least as important as temperature. Two field experiments with students⁴ showed that lower CO2 levels achieved through an open window or ventilation grille led to measurably better sleep and better concentration the following morning. An experiment in 29 bedrooms⁵ confirmed that better ventilation produces more deep sleep and fewer awakenings. In the older-adult study³, fresher air and a more comfortable temperature worked together: combined, they delivered 10 minutes more deep sleep and almost 4 minutes more REM sleep.

An observational study in which 62 people were followed for fourteen nights⁶ measured multiple factors simultaneously. Higher temperature, more fine particulate matter, higher CO2, and more noise were each independently associated with 3 to 5 percentage points lower sleep efficiency. Notably, participants appeared subjectively to adapt to poor conditions, while their objectively measured sleep still deteriorated. Humidity made no significant difference in this study.

Practically speaking: aim for 20 to 25 degrees and ensure ventilation -- a window left slightly open or a good ventilation system measurably lowers CO2 and improves your sleep. Temperature and ventilation reinforce each other. There are large individual differences: people who feel cold also sleep worse. The sweet spot is a well-ventilated, slightly cool room where nobody feels cold.