The short answer
Around 16C to 18C, which is cooler than most people keep a bedroom. Body temperature has to fall for sleep to start, and a warm room prevents it.
- Turn the bedroom radiator down rather than the whole house.
- Aim for 16C to 18C. If that feels cold, the answer is more bedding rather than more heating.
- A warm bath or shower an hour before bed helps, counter-intuitively, because the blood vessels in the skin dilate afterwards and body temperature drops faster.
- Feet out of the covers is a genuinely effective trick for the same reason.
What will ruin it
Do not let a bedroom get genuinely cold and damp to save money. A cold external wall with the door shut and two people breathing all night is the classic recipe for condensation and black mould in the corner.
Why it works
Sleep onset is triggered partly by a fall in core body temperature. A room that is too warm prevents the body from shedding heat, which is why people lie awake feeling uncomfortable without being able to say why.
This is general information, not medical advice, and it does not replace a GP, a pharmacist or NHS 111. Call 999 in an emergency. Full disclaimer.