British bedrooms are built to trap heat, not lose it, which is wonderful in January and miserable in a July heatwave. Brick walls and small windows soak up the day's warmth and release it slowly through the night, so the room is often at its hottest just as you are trying to drop off. Air conditioning is the obvious fix, but a portable unit is expensive to buy and one of the priciest things you can plug in. The good news is that most of the difference between a sweaty, sleepless night and a tolerable one comes down to timing, airflow and a few items you almost certainly already own. None of it costs much to run.
The short answer. You do not need air con to sleep in a heatwave. Shut windows, blinds and curtains during the day to keep the sun's heat out, then open up and create a through-draught once the outside air drops below the indoor temperature, usually late evening. A fan helps you sleep by moving air over your skin rather than by cooling the room, and a typical 50W fan costs only about 10p to run for an 8-hour night at 26.11p per kWh. Cooling your body directly, with a damp flannel, cool feet and light bedding, does more than trying to chill the whole room.
Keep the heat out during the day
The single biggest thing you can do happens hours before bedtime. On a hot day the air outside is often warmer than the air inside until early evening, so throwing the windows open at lunchtime simply invites the heat in. Sunlight pouring through glass is worse still, because it heats whatever it lands on and that warmth stays in the room long after the sun has moved.
Treat the bedroom like a flask. In the morning, close the windows and pull blinds, curtains or shutters across any window the sun will hit. Light-coloured or reflective curtains, or a blind on the outside of the glass, work best because they bounce heat away before it gets in. Keep internal doors shut so warm air from a sunny living room does not drift into the bedroom.
- Close windows and curtains on the sunny side first thing in the morning.
- Use pale or blackout-lined curtains, or foil-backed blinds, to reflect sunlight.
- Shut the bedroom door during the day to keep the cool air you have banked.
The same closed-up-by-day, open-by-night routine cools the whole home, not just the bedroom. There is more on the full-house approach in our guide to keeping cool without air con.
Open up at the right moment and make a through-draught
Once the outdoor temperature drops below the indoor temperature, usually mid to late evening on a UK summer day, it is time to flush the stored heat out. Opening a single window does very little. What moves real air is a through-draught: two openings on opposite or different sides of the home, so cooler air is pulled in one side and warm air pushed out the other.
If your bedroom has only one window, open it and open another window or door across the landing to create a path for air to travel. A fan placed in the window facing outwards, blowing the hot indoor air away, speeds up the exchange. Leave it going for an hour or two before bed and the room will feel noticeably fresher.
A cheap window or door thermometer takes the guesswork out of when to open up. If you are renting and cannot fit fixed shading, our notes on saving energy when renting cover what you can do without altering the property.
What a fan actually does, and what it costs
A fan does not cool the air. It moves it, and moving air across damp skin carries heat away by helping sweat evaporate. That is why a fan feels lovely when it is blowing on you and pointless when it is aimed at an empty corner. Point it at the bed, not the ceiling, and you get the benefit where it matters.
The cost is tiny. Fans draw far less power than people expect, so running one all night barely registers on the bill. Here is what common fans cost for an 8-hour night and over a month of nightly use.
| Fan type | Typical power | Cost per 8-hour night | Cost per month (nightly) |
|---|---|---|---|
| USB or small desk fan | 5W | about 1p | about 31p |
| Tower fan | 50W | about 10p | about £3.13 |
| Pedestal fan (full speed) | 70W | about 15p | about £4.38 |
At 26.11p per kWh, Ofgem cap July to September 2026. Wattages are typical; check the label on your own fan.
Even the thirstiest fan here costs less for a whole month of nights than a single hour or two of a portable air conditioner. For the full breakdown by speed and fan size, see how much a fan costs to run. If you are weighing up buying a cooling unit, our comparison of a portable air conditioner versus a fan is the place to start.
Cool your body, not the whole room
You do not need to chill the entire room to sleep. You need your body to lose heat, and your body sheds most of its heat through the hands, feet, face and the points where blood runs close to the skin. Targeting those spots is faster and cheaper than cooling the air.
- Run cold water over the insides of your wrists, or hold a cold pack against them, before bed.
- Keep your feet out from under the covers, or stick them out of the side of the bed. Cool feet help the whole body cool down.
- Have a lukewarm shower rather than a cold one. Cold water makes your body try to retain heat; lukewarm lets it carry heat away as the moisture evaporates.
- Lay a damp flannel or a frozen water bottle wrapped in a tea towel within reach for the small hours.
- Swap to cotton or linen sheets and a thin duvet or just a sheet. Synthetic bedding traps heat and sweat.
An old camping trick worth knowing: a shallow tray of ice or a couple of frozen water bottles placed in front of a fan gives a brief blast of genuinely cooler air. It will not last all night, but it helps you get to sleep.
Why air con is the last resort, not the first
It is tempting to solve the problem by plugging in a portable air conditioner, and on the worst few nights of the year some people do exactly that. The catch is the running cost. A portable unit typically draws 1 to 1.5 kW, far more than any fan, so an 8-hour night sits in the region of £2 to £3 rather than a few pence. Across a hot fortnight that adds up quickly.
There is a practical snag too. A single-hose portable air conditioner has to vent its hot exhaust outside through a window kit, and if that hose is not sealed properly it pulls warm air back in and fights itself. Many people end up running them with the window cracked open and wonder why the room never gets cold. If you do go down this route, our guide to air conditioning running costs sets out what to expect on the bill, and our piece on keeping cool without air con covers the cheaper alternatives first.
For most UK homes, on most hot nights, the fan-and-airflow approach gets the room comfortable enough to sleep without the cost or the hassle of a unit you only use a handful of nights a year.
The bottom line
Sleeping through a heatwave is mostly about timing and your own body, not machinery. Keep the sun and warm air out during the day, flush the heat with a through-draught once it cools outside, and use a fan to move air over your skin while you cool your wrists, feet and face directly. Light bedding and a lukewarm shower finish the job.
A fan for the whole night costs around 10p, while a portable air conditioner can cost twenty to thirty times more for the same hours. Reach for the cheap fixes first. On all but the most extreme nights they are enough, and they cost almost nothing. For the daytime version of all this, our summer energy checklist pulls the cooling tactics together.