How are you supposed to get 40 winks when it feels like the temperature in your bedroom is pushing 40°C (104°F)? As our climate warms and intense heatwaves become more common, scientists note that temperatures are rising faster at night than during the day.
It means climate change is something to lose sleep over, literally. Around 44 hours of lost sleep is attributed to heat each year, and it’s happening everywhere. Earlier this year, a team of researchers published the first planet-scale evidence that warmer temperatures are eroding human sleep.
The study covered 68 countries, using wrist-based sleep trackers to compare 47,000 adults’ sleep not just against averages, but against their own personal baselines. Unsurprisingly, the hotter it was, the less people slept.
“People fell asleep later and woke up earlier than they normally would,” says Dr Kelton Minor, a data scientist at Columbia University and the study’s lead author. “This increased the probability that people had a short night of sleep, sleeping less than seven, six, even five hours compared to their individual baseline.”
The research suggests that by the end of the century, each of us could face an average of two weeks of temperature-related short sleep per year, or between 50–58 hours of sleep loss. Those aren’t the kind of numbers you just catch up on.
“You might suspect that if you sleep worse on a really hot night, you’ll catch up the next night, or over the next week, or nap during the day,” says Minor. “We were really surprised: regardless of the timescale, people weren’t adapting well to increasing nighttime temperatures.”
Bags under our eyes are the least of our worries. Lack of sleep is associated with a hot soup of acute and long-term health issues, not to mention lower mood, cognitive ability and productivity. A 2022 study published in the journal Lancet Planetary Health, found that the risk of death from hot nights could increase six-fold by the end of this century.
So what can we do? Minor says that while air-con is life-saving technology, adding it to every home would take a huge toll on global energy expenditure. And although it cools a building, it simply transfers the heat outside, rather than actually lowering temperatures.
There are fans, of course. They use up to 50 times less electricity than AC and an article in the Lancet states that they “accelerate convective and evaporative heat losses from skin.” But if the temperature creeps above 35°C (95°F), turn it off before it becomes more like a hair dryer.
Read more:
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- What does sleep do for the brain?
- Regular afternoon naps linked to improved cognitive function
Most sleep research promotes an optimal indoor climate of around 19°C (66°F) to support the natural drop in our core body temperatures that accompanies good sleep.
During heatwaves, it’s best to leave the windows closed to prevent hot air from intruding, while shutting the blinds or curtains will deflect heat. If the outdoor temperature drops, fling them open and create a cross-breeze by opening doors or windows on opposite sides of your home.
Lean into salads, too. Yes, they’re full of healthy and hydrating foods anyway, but they also keep you away from the hob, which increases the indoor humidity.
That Lancet paper details a long list of steps to keep cool on hellish nights, such as hanging a wet sheet in front of an open bedroom window to create a misting effect, or spraying yourself with cool water. Doing this repeatedly can reduce dehydration by 65 per cent in hot, dry conditions, while sitting in front of a fan at the same time cools you faster by increasing evaporation.
Taking a cool shower before bed increases the rate at which your body sweats away the heat, or you could sink your hands and feet into a cold bath to lower your core body temperature. A study on joggers made to run in 39°C (102°F) conditions, found that chest immersion in cool (not cold) water brought temperatures down faster.
It also makes sense to follow good general sleep habits. Although, Minor warns “there’s a lack of evidence about whether sleep hygiene related to say, light exposure or alcohol avoidance, helps to reduce the negative effect of temperature.”
If all else fails, companies have started selling what amounts to an anti-electric blanket: mattress toppers that are blasted with cool air while you sleep. With the nights getting hotter, it could be a choice between that or smuggling ice packs in your PJs.
About our experts
Dr Kelton Minor is a data scientist at Columbia University and studies how we are adapting to climate and technological changes. His work has been published by One Earth, The Lancet, and Sleep Medicine Reviews.
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