The ideal bedroom is cool — around eighteen degrees — with humidity between forty and sixty per cent. The coolness helps your core temperature make the 1.2-degree drop needed to fall asleep; the right humidity keeps your airways and skin settled and prevents mould.
Your body starts sleep by giving off heat through the skin; a cool environment speeds that up. Around eighteen degrees works best for most people. Sleeping too warm suppresses both deep sleep and REM and produces restless nights.
The material of your bed counts more here than most people think. Natural fibres such as wool, cotton and horsehair carry moisture away and let heat through; closed materials hold on to it. You won't notice that difference in ten minutes, but you will at four in the morning.
Air that is too dry, below forty per cent, irritates airways and skin. Too humid, above sixty per cent, feels close and encourages dust mites and mould. Between those two values lies a wide and comfortable range.
In our houses winter is the problem: heating dries the air out sharply. A hygrometer costing a few tens of euros tells you within a day whether this applies to you.
Not for the room, yes for you — and that is exactly the point. You cover yourself up; it is your surroundings that must be cool so your body can shed heat. Anyone who finds it too cold usually has bedding that is too warm, not a room that is too cold.
You might be. Materials that respond to heat, such as memory foam, hold on to it; natural fibres carry moisture and heat away instead. Anyone waking at night because they are overheating should look there first, not at the thermostat.