When a heatwave arrives, one of the first things many people will notice is the negative impact on their sleep. However, this is not just discomfort. Heat interferes with the biology that allows us to fall asleep and stay asleep. Understanding why can make the practical steps that help far easier to stick to.
Sleep is closely tied to body temperature. Over each 24-hour cycle, our core temperature rises and falls in a predictable rhythm, and the temperature begins to drop in the evening as part of the signal to the body that it is time to sleep. In preparation for sleep, the body pushes heat to the extremities and blood vessels in the skin widen to release heat, which helps lower core body temperature.
This is why the temperature of your surroundings matters so much at night. External temperatures can interfere with the natural thermoregulation that occurs during sleep. If your bedroom is too warm, this can raise your body temperature and disturb your sleep. When the air around you is hot, your body cannot shed heat as easily and the cooling process that sleep depends on is disrupted.
The impact of heat on sleep
The effect of a hot room is not only that it takes longer to drop off. Heat changes the structure of sleep itself. Research suggests heat exposure increases the number of night-time awakenings and reduces both REM sleep and slow-wave sleep (the deep stage in which the brain and body recover).
Deep sleep and REM sleep are the most restorative stages, so losing them is why you can spend eight hours in bed on a hot night and still wake feeling unrefreshed. There is also a compounding effect as disrupted sleep can unsettle your body clock and make the following nights harder too, leaving people feeling progressively more exhausted.
The bedroom temperature
There is a broadly agreed comfortable range for sleep. The Sleep Charity and NHS guidance suggest an ideal bedroom temperature of around 16 to 18 degrees, with anything over 24 degrees tending to cause restlessness. International sleep bodies converge on a similar range, so this is a well-established figure rather than a rule of thumb.
How to cool the room
Keeping curtains or blinds closed while the sun is on the windows stops a lot of heat building up in the first place. It also helps to keep windows shut during the hottest part of the day, then open them once the evening air outside is cooler than the air inside.
Creating airflow makes a real difference once temperatures drop. Opening windows on opposite sides of your home allows cooler air to move through rather than sit still. Moving air helps sweat evaporate from the skin, which is one of the body’s main cooling mechanisms.
How to bring your own body temperature down
A cool (not cold) shower before bed is a simple option. Running cool water over your wrists or splashing your face can also help cool down. Cooling the feet is also effective as keeping them outside the duvet/covers lets heat escape through the skin.
Light, loose, natural fabrics such as cotton allow air to circulate and absorb moisture better than synthetic materials. While counterintuitive, wearing light nightwear can be more comfortable than none, as it helps wick sweat away from the skin.
A warm bath or shower taken roughly 60 to 90 minutes before bed can also aid sleep, as raising skin temperature prompts the body to cool afterwards through the widening of blood vessels.
Use of fans to cool down
A fan is the most common tool people reach for and for most of a British summer, it genuinely helps to cool you by moving air across the skin and speeding up the evaporation of sweat. Positioning it to draw cooler air in from outside (or to create a through-draught) tends to work better than simply pointing it at the bed.
However, once the surrounding air is hotter than your skin (at around and above 35 degrees), a fan begins to blow hot air over you and can add heat rather than remove it. In those conditions, a fan works far better combined with wetting the skin. However, in the UK, such temperatures are rare overnight, so for most people a fan still remains useful.
What to drink
Staying hydrated through the day helps your body cope with heat. It is recommended to keep a glass of water by the bed, but best to avoid drinking large amounts of water in the hour before sleep.
While alcohol can feel relaxing and may help you drop off, it fragments sleep later in the night and raises your heart rate. Caffeine also lingers in the body for several hours after drinking, which can affect your ability to fall and stay asleep.
Health concerns
While most disturbed sleep in hot weather is uncomfortable rather than dangerous, it is worth knowing the warning signs of something more serious. Heat exhaustion can cause tiredness, dizziness, headache, feeling sick, heavy sweating and intense thirst, and it is not usually an emergency if the person can be cooled and given fluids and starts to feel better within 30 minutes.
Heatstroke, however, is a medical emergency and needs a 999 call. The warning signs include feeling unwell after 30 minutes of cooling and rest, a very high temperature, hot skin, confusion, a seizure or loss of consciousness.
