Why can't you sleep when it's hot? Because your body literally can't. Being too hot is the leading cause of disrupted sleep in the UK. The Dreams 2024 survey of 15,000 adults found that 37% of people cite overheating as their main sleep disruptor, ahead of stress, noise, and light. And only about 5% of UK homes have air conditioning. So when summer arrives or your bedroom sits above a boiler, you're fighting biology with nothing but a thin sheet and an open window.
What happens to your body temperature at night
Your core body temperature needs to drop by about 1-1.5°C to initiate sleep. This isn't optional. It's a hard requirement built into your circadian rhythm. Your body starts this cooling process in the evening, pushing heat from your core to your extremities (which is why your hands and feet warm up before you feel sleepy).
If your bedroom is too warm, your body can't dump that heat fast enough. You lie there alert, restless, kicking the duvet off then pulling it back on. Even once you fall asleep, a warm environment reduces time spent in deep sleep and REM, the stages where physical restoration and memory consolidation happen.
The optimal bedroom temperature for sleep is around 18-20°C according to most sleep research. The NHS says "keep your bedroom cool" without specifying a number. Above 24°C and sleep quality drops measurably in most people.
Why some people run hotter than others
Metabolic rate varies. People with higher muscle mass generate more heat at rest. Hormonal changes (menopause, thyroid conditions, pregnancy) raise core temperature. Some medications increase body heat as a side effect. And some people are just naturally warmer sleepers without a clear medical reason.
If you've always run hot, the environment matters more to you than to someone who sleeps cold. The bedroom cooling strategies need to be more aggressive.
What to do when you can't sleep because it's hot
Ranked by impact per pound spent:
Switch to a lower tog duvet. If you're under a 10.5 tog in summer, that's your problem. Drop to 4.5 or use a flat sheet. Our best duvet for hot sleepers guide covers tog ratings and fill materials.
Change your sheets. Polyester traps heat. Cotton percale, linen, or bamboo breathe. This costs under £30 and makes a noticeable difference the first night.
Block the sun during the day. Close curtains or blackout blinds on sun-facing windows. A south-facing bedroom with open curtains can be 5-8°C warmer by evening than one with curtains closed.
Get a fan. Moving air accelerates sweat evaporation, your body's main cooling mechanism. A fan doesn't lower room temperature but it lowers your perceived temperature by 3-4°C.
Cool your pillow. Your head generates a lot of heat. A cooling pillow or even just putting your pillowcase in the freezer for 20 minutes helps you fall asleep faster.
For the full list of bedroom cooling strategies, our how to cool your bedroom without AC guide has 9 methods ranked by effectiveness.
When to address the mattress
Memory foam mattresses trap heat. That's the trade-off for the contouring support. If you're on memory foam and overheating, a cooling mattress topper adds a breathable barrier. Gel-infused and open-cell foam toppers are the most effective.
When heat disruption might be medical
Persistent night sweats that soak your sheets regardless of room temperature could indicate something beyond a warm bedroom. Hormonal imbalances, infections, anxiety disorders, and some medications all cause excessive sweating at night. If cooling your environment doesn't help and you're consistently drenching your bedding, speak to your GP.
FAQ
What temperature is too hot to sleep? Above 24°C gets uncomfortable for most people. Above 26°C and sleep quality drops significantly. The optimal range is 18-20°C. A room thermometer costs a few pounds and tells you exactly where you stand.
Why do I overheat in bed but not during the day? Your mattress and duvet trap the heat your body generates. During the day you move around and air circulates. In bed you're lying still in a heat cocoon. The insulation around you amplifies your body heat.
Does a cold shower before bed help? Counterintuitively, no. A cold shower constricts blood vessels and causes your body to retain heat. A lukewarm shower dilates blood vessels and helps your body release heat naturally.
Can being too hot cause nightmares? There's some evidence that thermal discomfort increases vivid or disturbing dreams, though the mechanism isn't fully understood. At minimum, overheating causes more fragmented sleep with more awakenings, which means you're more likely to remember dreams (including unpleasant ones).
