Hot nights and can't sleep: what can I do?

If you can't sleep on hot nights, the heat is the first thing to look at: core body temperature falls at night, sleep comes most easily while it is falling, and a hot, humid bedroom makes it harder for the body to lose heat. Large studies link warmer nights with more nights of too little sleep, and a review of real-world research found that higher outdoor and indoor temperatures are generally linked with poorer and shorter sleep worldwide. What you can do: keep the sun out of the bedroom during the day and let cooler air in at night, use a fan only when the room is below about 32 to 35°C and air conditioning when it is hotter, keep bedding and clothes thin, drink regularly through the day, and skip alcohol and late caffeine.

This article is about sleep on hot nights. For feeling drained and off your food during hot days, see Drained and off your food in hot weather? What can I do?

Why is it hard to sleep when it's hot at night?

Body temperature falls at bedtime

Body temperature follows a daily rhythm. A 2012 review by Okamoto-Mizuno and Mizuno, sleep researchers in Japan, explains that core body temperature falls during the night-time sleep phase, and that sleep is most likely to start while it is falling. To lose that heat, the body sends warm blood to the skin. When the air around you is hot, the heat has nowhere to go.

The review adds that in real-life conditions, with bedding and clothes, heat increases the time spent awake and reduces both deep sleep (slow-wave sleep) and REM sleep.

Humidity makes it worse

Thai nights are often humid as well as hot. The same review notes that humid heat further increases the heat load on the body during sleep. When the air is dry, sweat evaporates and carries heat away. When the air is humid, sweat cannot evaporate, the skin stays wet, and the body has a harder time cooling down. The authors note that this matters especially in Japan and other Asian countries with humid summers.

Heat hits the first part of the night hardest

The review reports that the effects of heat on sleep stages are concentrated in the first part of the night rather than the later part.

How much does heat affect sleep, and is Thailand getting hotter?

What studies around the world have found

Most of these are observational studies, many of them in cooler countries, and the US survey relied on people's own reports of their sleep. Even so, they point the same way: hot nights tend to go with poorer and shorter sleep.

What Thai data show about the heat

The 2024 study notes that most of the upward trends were not statistically significant, and neither study reports night-time temperatures separately. What they do show is that maximum temperatures rose between 2001 and 2016, and that humidex rose significantly in the south and east, so the bedroom is worth setting up for hot, humid weather.

How should I set up the bedroom on a hot night?

During the day: keep the heat out

The UK's NHS advises closing windows, curtains and blinds during the day and opening them at night when the temperature outside has gone down. In a Thai home, that means drawing the curtains in a west-facing bedroom before the afternoon sun hits it, so that the room is not holding the day's heat at bedtime.

At night: let cooler air in

Once it is cooler outside than inside, open windows to let air move through, if it is safe to do so. The US CDC advises keeping the bedroom quiet, relaxing and at a cool temperature as part of good sleep habits for everyone.

Fan or air conditioning?

The two agencies draw the line at slightly different temperatures, so on the hottest nights, when the room stays above about 32°C, a fan blowing hot air is not enough. The CDC suggests air conditioning, or finding a place that has it, in very hot weather.

The 2012 review notes that in Japan most people run the air conditioner for only a few hours after going to sleep. If you set a timer, keep in mind that the same review found heat disturbs the first part of the night most.

Bedding and clothes

The review reports that people sleep normally when the small pocket of air between the body and the covers is around 32 to 34°C with 40 to 60% relative humidity. On a hot, humid night that pocket gets hotter and wetter quickly. Thin, breathable sheets and loose nightclothes let sweat evaporate more easily than a thick blanket or tight synthetic clothing.

What can I do in the evening before bed?

Cool down your body

The NHS suggests a cool shower, or putting cool water on your skin or clothes, to cool down in hot weather.

If you still feel hot when you go to bed, a cool shower is one option.

Drink through the day, not all at bedtime

The NHS suggests 6 to 8 cups or glasses of fluid a day as a general guide, more in hot conditions, and aiming for urine that is a clear pale yellow. Spreading drinks across the day means you are less likely to go to bed thirsty, or to drink a lot just before sleep and wake up for the toilet.

Go easy on alcohol, caffeine and late meals

An ice-cold beer feels refreshing on a hot evening, but the NHS and CDC both advise against alcohol in the heat and before sleep. For broader habits that help sleep, see Want to sleep better? Which habits can you change?

Who needs extra care on hot nights?

The NHS advises checking the temperature of rooms, especially where people at higher risk live and sleep. The US CDC advises checking on family, friends and neighbours, especially those who live alone. The US study above found the effect of hot nights on sleep was largest among older people and those on lower incomes.

• Older parents: call on very hot evenings to ask how hot the bedroom is and whether they have had enough to drink. For other reasons older parents wake at night, see Older parents waking up at night: how can family help?
• Young children: they cannot tell you clearly that they are too hot, so check the room and their skin.
• People who work in the heat all day: they may start the night already hot and low on fluids. See Sweating all day working outdoors: what should I drink?
• Anyone taking regular medicines: ask a doctor or pharmacist whether you need to take extra care in hot weather, and do not stop a medicine on your own.

When is feeling hot at night a warning sign?

Signs to act on now

Feeling too hot to sleep is common. Feeling unwell from the heat is different. The NHS lists signs such as tiredness, dizziness, a headache, feeling or being sick, heavy sweating, pale and clammy skin, cramps in the arms, legs or stomach, a high temperature and being very thirsty. If you notice these, the NHS advises:

1. move to a cool place
2. take off unnecessary clothing
3. drink plenty of water
4. cool the skin with cool water and fan it

The NHS says people should start to feel better within 30 minutes.

Call for help straight away

In Thailand, call 1669 straight away if someone has a very high temperature, hot skin that is not sweating, a fast heartbeat, fast breathing or shortness of breath, confusion, a fit or loss of consciousness, or is still unwell after 30 minutes of resting somewhere cool.

If poor sleep goes on

If you keep sleeping badly for weeks, even on cooler nights or in a cooler room, and it affects how you feel during the day, talk to a doctor. Before you start taking anything new to help you sleep, check with a doctor or pharmacist.