The mechanism, in one sentence
An hour before sleep onset, the body dilates the vessels of the hands and feet in order to shed heat. Core temperature then falls by three tenths of a degree to a full degree, and that fall is the signal for entering sleep. Anything that prevents it delays onset: a room that is too warm, a duvet that is too thick, a mattress that stores heat, a scalding shower taken ten minutes before bed.
It is a physical mechanism, measurable, and indifferent to willpower. It also explains why some sleepers open the window in midwinter, which is not an aesthetic preference.
What the benchmarks are worth
A range of 16 to 19 degrees is commonly quoted for a bedroom. The figure is useful but incomplete, because it means nothing without the bedding. What counts is the balance between the heat produced, the insulation provided by duvet and clothing, and the room’s ability to shed it. Two degrees less under a winter duvet is not the same as two degrees less under a sheet.
Three things deserve more attention than the thermostat itself.
Relative humidity, first: above 60 per cent, evaporation from the skin becomes ineffective and 22 degrees turns unpleasant, while the same 22 degrees at 45 per cent passes unnoticed.
Surfaces, next. A room at 20 degrees whose north wall sits at 16 produces a radiant discomfort that the air thermometer never sees. The reverse happens in summer, with a poorly insulated roof radiating all night.
Stability, finally. A bedroom that climbs three degrees between midnight and six disturbs sleep more than a stable bedroom that is slightly warmer.
What hot nights show
Night-time heat episodes provide a natural experiment. Work carried out on very large volumes of data from wrist-worn devices shows, above night-time temperatures of roughly twenty-five to thirty degrees, a shortening of sleep duration and an increase in awakenings. The effect is more pronounced in older people, in women, and in the hottest regions.
These data are observational and rest on indirect measures of sleep. They nonetheless agree with laboratory studies, where ambient temperature is manipulated without the sleeper’s knowledge: deep slow-wave sleep and REM sleep both decrease when the room is too warm. The direction is solid. The exact magnitude is less so.
The number of so-called tropical nights, those that never fall below twenty degrees, is rising. That is a design constraint, not an individual failing.
What can be corrected, in order
Shade outside before shading inside. A shutter, an external blind or a brise-soleil stops radiation before it enters. An interior curtain absorbs the heat once it is already in the room, and gives it back. The difference between the two is counted in degrees, not tenths.
Ventilate at night and close during the day. In a building with thermal mass, thick walls or a concrete slab, that discipline alone often carries the summer. It assumes one can open up without noise or insects, which points back to the façade and the joinery.
Deal with the bedding. A memory foam topper can add the equivalent of one to two perceived degrees. Natural fibres shed vapour better. This is the cheapest and the most immediate correction.
Take a warm bath or shower one to two hours before bed. The result looks counter-intuitive: warming the skin triggers vasodilation, which then accelerates the loss of core heat. Available syntheses report sleep onset shortened by around ten minutes on average. The effect depends on timing: too close to bedtime, it reverses.
Air-condition last, and properly. A badly placed unit creates a cold draught over the bed, a continuous noise of around thirty-five decibels and a drying of the air. Where it is justified, the questions are the position of the outlet, the sound level of the indoor unit and the night-time setpoint, not the capacity.
What we do
We record temperature and relative humidity at the sleeping point over several nights, we measure surface temperatures when discomfort cannot be explained by the air alone, and we establish what belongs to the building, to use, and to the bedding. Lif8 measures places: we are not healthcare professionals, and persistently unrefreshing sleep is a matter for a doctor.
