Sleep science
Why do you get hot when you sleep? Causes and fixes for night sweats
Waking up hot is not a character flaw of your bedroom — it is a heat-management problem, and it usually has one of a handful of causes. Some are cheap to fix tonight, some take a bedding change, and a few are worth mentioning to a doctor. Here is how to tell which one is yours.
1. The air under your duvet is saturated
The single most common reason people overheat in bed has nothing to do with room temperature. Under a duvet, the air against your skin climbs toward body temperature and near-100% humidity. Once that happens your sweat can no longer evaporate — and evaporation is the body's main cooling method during sleep. You are effectively lying in your own trapped humidity.
This is why kicking the duvet off feels instantly better but freezing ten minutes later: you fixed the blanket, not the humidity. The fix is air movement under the covers, not a colder room — moving air carries that saturated layer away so sweat can evaporate again.
2. The bedroom is above the sleep window
Your core temperature has to fall by about 1°C for sleep to begin, and research consistently favours a bedroom around 16-19°C (60-67°F). Above that range sleep gets lighter and more fragmented. A thermostat reading of 21°C can still mean a 27°C+ microclimate under a thick duvet with a memory-foam mattress.
Set the thermostat toward the lower end of the window, but do not chase a cold room as the whole answer — airflow under the covers usually matters more than two degrees on the dial.
3. Your mattress and bedding trap heat
Memory foam is the usual suspect: it conforms closely, which limits airflow around your body and stores heat. Microfibre sheets behave similarly, while cotton percale, linen and bamboo blends breathe noticeably better. A heavy winter duvet left on into spring compounds all of it.
If you wake damp but the room feels fine, bedding is the most likely cause — and the cheapest to change. Swap to breathable sheets and a lower-tog duvet before you buy anything else.
4. Hormones: menopause, cycles and medication
Night sweats driven by hot flushes (commonly in perimenopause and menopause) behave differently from ordinary overheating: they arrive as sudden flushes, often in the second half of the night, and no thermostat will prevent them. The realistic goal is managing the symptom — keeping sheets dry so a flush does not turn into a full bed change at 4am.
Some medications, low blood sugar at night, infections and thyroid issues can also cause night sweats. If the sweating is new, severe, or comes with other symptoms, talk to a doctor — it is worth ruling out other causes first.
5. Evening habits raise your starting temperature
Alcohol close to bedtime widens blood vessels in the skin and makes you feel hotter hours later, even though it makes you drowsy first. Late intense exercise, big late meals and hot showers straight before bed all raise the temperature your body starts the night from.
A warm (not hot) shower 60-90 minutes before bed does the opposite: it pulls blood to the skin, accelerates the core temperature drop, and most people fall asleep faster for it.
6. Two people, one climate
Couples rarely run at the same temperature, and a shared duvet means whoever runs hotter loses. If one of you wakes up damp while the other sleeps fine under the same blanket, that is the strongest possible clue the problem is local airflow, not the room.
This is the case where cooling one side of the bed — rather than the whole room — ends the thermostat argument entirely.
What actually works, in order
Most hot sleepers who fix the problem stack three or four small changes rather than one dramatic one. From cheapest to most involved:
- Breathable bedding: cotton percale or linen instead of microfibre, and a lower-tog duvet in warm months
- A warm shower 60-90 minutes before bed to trigger the core temperature drop
- Cooler thermostat setting, toward the low end of the 16-19°C window
- Directed airflow under the covers — a quiet bed fan rather than a fan blowing at your face
- Less alcohol and late exercise, especially on the worst nights
Where a bed fan fits
A bed fan solves the specific problem in cause 1: it slides gentle airflow into the gap under your duvet and continuously replaces saturated air with drier room air. It uses a fraction of the energy of air conditioning, runs quiet enough to sleep through, and cools one side of the bed without touching the other.
It will not lower a sweltering 30°C room on its own, and it does not treat hormonal causes — it manages the overheating so you stay asleep. That combination, plus breathable bedding, is what ends the 3am wake-up for most people.
The takeaway
You get hot at night because heat and humidity get trapped where you sleep — under the covers — faster than your body can shed them. Fix the airflow and the bedding first, keep the room toward the cool end of the window, and treat night sweats that arrive suddenly or severely as something to discuss with a doctor.