The three bedroom numbers worth knowing: temperature, noise, light

A bedroom can be improved endlessly, or it can be improved on three measurements. Temperature, noise and light are the ones with published figures behind them, and they are the ones most often set wrong by a wide margin.

Temperature: colder than most rooms are kept

The Sleep Foundation recommends about 18.3 °C, within a range of 15.6 to 20 °C. The INSV states that the temperature is optimal between 18 and 19 °C and should not exceed 20 °C. Both land in the same place, and both are below what most bedrooms actually sit at in winter.

The reason is physiological rather than a matter of comfort: falling asleep follows a drop in core body temperature, and a room that is too warm works against it. A bedroom heated to 21 or 22 °C is not cosy from the point of view of sleep onset — it is an obstacle.

Noise: forty decibels, and it is a ceiling not a target

The World Health Organization's recommendations for Europe put the desirable night-time level below 40 dB(A). That figure is not an ideal: it is the lowest level at which adverse effects have been observed. Below it, effects are not demonstrated; above it, they begin to be.

Forty decibels is quieter than it sounds — roughly a quiet library, well below a conversation in the next room or a main road behind a closed window. For anyone living on a street, reaching it by insulation alone is rarely realistic, which is why masking rather than silencing is the more common answer.

On that point the evidence is real but modest. A systematic review published in Sleep Medicine Reviews in 2021, covering thirty-eight studies, examined noise used as a sleep aid. It is worth knowing that the effect exists and is not spectacular — a useful corrective to the claims made for white-noise devices.

Light: a few lux, not a few hundred

This is the figure that most often surprises. Inserm underlines that it was long believed more than a thousand lux were needed to affect the system, but that a few lux are enough — the brightness of a full-moon night can block the secretion of melatonin.

A standby LED, a street lamp through thin curtains, a phone screen consulted at three in the morning: each is comfortably above a few lux. And the timing matters as much as the intensity. Inserm specifies that late exposure, in the evening, delays the clock, while morning exposure advances it. The same light has opposite effects depending on when it lands.

Caffeine, since it belongs to the same conversation

The reference study on the question compared 400 mg of caffeine — about two to three cups of coffee — taken zero, three and six hours before bed. The finding that made it a reference is that even the six-hour dose measurably disturbed sleep. The practical rule that follows is a cut-off time rather than a ban.

What this adds up to

Three numbers, none of which require a specialist: a room between 18 and 19 °C, a night-time level under 40 dB(A), and as close to darkness as the room allows. They are cheap to check and they are wrong in most bedrooms — which is precisely why they are worth checking first.

Sources: Sleep Foundation; INSV; World Health Organization, Night Noise Guidelines for Europe; Inserm; Sleep Medicine Reviews, 2021.