THE SHORT ANSWER
Around 18 °C (65 °F), with a healthy range of roughly 15.6-19.4 °C (60-67 °F). Sleep onset depends on your core temperature dropping about 1 °C, and a cool room lets that happen. Most people who "sleep badly in summer" are simply too warm - and older adults and babies need slightly warmer rooms.
Your core temperature naturally falls in the evening and bottoms out a couple of hours before you wake - the drop is part of the sleep signal itself [1]. You shed that heat through your hands and feet, which is why cold feet keep people awake: the body can't offload heat through constricted blood vessels. A cool room makes the whole process easier; a warm one fights it, and warmth specifically cuts into deep sleep and REM [2].
This also explains the bath trick: a warm bath 1-2 hours before bed brings blood to the skin, so you dump heat afterwards and your core drops faster. A meta-analysis found it shortens time to fall asleep by around 10 minutes [3].
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Set the thermostat to 18 °C - or as close as your heating and bills allow. A 2 °C drop is usually noticeable within a few nights.
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Warm bath or shower, 1-2 hours before bed - the post-bath heat dump speeds your core temperature drop - about 10 minutes off sleep onset in pooled trials [3].
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Warm feet, cool room - socks in bed sound counterintuitive but help: warm extremities dilate blood vessels so the core can shed heat faster.
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Breathable bedding over fewer layers - cotton or linen and a lighter duvet beat cranking the AC. Humidity matters as much as air temperature.
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Sleep two duvets if you share a bed - the Scandinavian setup. Thermal comfort differs sharply between people; one duvet forces a compromise nobody sleeps well in.
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Cool the room before you get in - window open or fan on an hour ahead. Falling asleep is the temperature-sensitive part - get it right at the start of the night.
18 °C is the average. Find your own.
Thermal comfort is genuinely personal - bedding, humidity, hormones and a shared bed all move the number. The honest test: drop your bedroom 2 °C for 14 nights, log restfulness, compare the averages. Claritips does the arithmetic: "restfulness in a cooler room: 4.0★ vs 3.2★ before."
Run the 14-night test - free → Is 22 °C too warm to sleep?
For most adults, slightly - you’ll likely still sleep, but with lighter deep sleep and more awakenings. It’s well within the right range for a baby’s room, though.
Why do I sleep badly in summer?
Warm nights blunt the core-temperature drop that sleep depends on, so onset takes longer and deep sleep suffers. Cooling the room before bed, lighter bedding and a pre-bed shower recover most of the loss.
Should I sleep with a fan on?
It helps two ways: air movement aids heat loss, and the steady noise masks disruptive sounds. Watch for dryness - a dry throat or eyes means point it away from the bed.
Does room temperature affect night sweats?
It contributes, but hormonal causes (menopause, thyroid), alcohol and some medications matter more. If night sweats are new or drenching, that’s a clinician conversation, not a thermostat one.
What if my partner and I disagree?
Set the room cool and adjust with bedding - separate duvets, different tog ratings. That’s easier than negotiating a thermostat, and cooler-plus-warmer-bedding beats warmer-plus-lighter for both of you.