The downside of hot summer days are hot summer nights. When the temperature doesn’t drop below 20 degrees Celsius (68 degrees Fahrenheit) at night—as is currently the case in many parts of Europe and North America—we become restless. We toss and turn in bed for hours, find it difficult to fall asleep, and feel groggy the next day. Sound familiar?
This has mainly to do with how closely sleep and the body’s temperature regulation are linked. Our internal temperature, which is normally around 37 degrees Celsius, naturally drops a little at night to make us fall asleep. About 1 degree of heat is redistributed from the core of the body to the hands and feet, which have large surface areas and specialized blood vessels to allow this heat to dissipate. The hormone melatonin plays an important role in this: When it’s dark, melatonin is secreted from the pineal gland in the brain and serves as a timer for our internal clock. It widens the blood vessels in the hands and feet to allow the body to rid itself of heat faster and help us nod off.
That is, if the ambient temperature doesn’t mess things up. The ideal bedroom temperature for adults is somewhere between 15 and 19 degrees Celsius (59 and 66 degrees Fahrenheit), depending on the person, and the body has to work harder to regulate its own temperature when this isn’t achieved. And if the room temperature doesn’t fall sufficiently after a hot day, then our ability to regulate our body temperature is impaired. Not only do we then have trouble falling asleep, but the hot air can interrupt our sleep stages too.
Our brain cycles through four stages of sleep—awake, light, deep, and rapid-eye movement (REM) sleep—for an average of 90 minutes, repeating the cycle four to six times each night.
Deep sleep is particularly important. During this stage, breathing and brain activity slow down, with the brain using this time to form and consolidate memories. It’s also this sleep stage that leaves us feeling refreshed. Unfortunately, it is particularly sensitive to temperature.
“We know that cooler temperatures support deep sleep,” says Christine Blume, a sleep scientist at the University of Basel in Switzerland. So when our ability to regulate body temperature is impaired because it is too warm, this leads to us not getting into the deep-sleep phase, she explains. “And if deep sleep is missing, then we simply lack rest,” she says.
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Sleep in a hot room and the fourth stage of sleep might be disrupted too. A 2020 study found that higher bedroom temperature is also associated with a shorter duration of REM sleep. When REM sleep is interrupted, the sleep cycle has to start over again. The exact role of REM sleep is still under debate, but it’s been hypothesized to play a role in memory formation, learning new motor skills, and regulating emotions.
Being sleep-deprived over the course of several days can affect your mental state and cause you to be irritable and angry, says Michelle Miller, an associate professor of biochemical medicine at the University of Warwick. “In a heat wave, I would be more concerned about short-term effects, such as cognitive function, impaired performance and judgment, and mood changes,” she says. People who plan to drive or who work in high-pressure occupations where cognitive function is important—such as police or health services, finance, or professions that involve operating machinery—should be especially aware of these effects, she adds.
Getting less than seven hours of sleep a night regularly, the minimum benchmark for adults, has also been associated with heart problems, obesity, and type 2 diabetes, among other conditions. “People try to do short sleeps during the week and then catch up on the weekend, but you never fully catch up on the health and cognitive benefits of sleeping properly throughout the week,” says Miller.
Hot, sleepless nights also aren’t a particularly new problem. A recently published study estimates that in 2010, each person across the globe was already losing on average 44 hours of sleep per year because of hot nighttime temperatures. As a result of these lost hours, on average adults were experiencing 11 additional nights each year when they got less than seven hours of sleep.
As air temperatures continue to rise, people could be missing out on even more. The same study—which linked the sleep-tracking wristbands of more than 47,000 people in 68 countries to local meteorological data—predicted that people could be losing 50 hours of sleep per year by the end of the century. Six additional lost hours spread over the year may not seem like much, but this would result in around 13 additional short nights of sleep, which is hardly welcome.
The study’s researchers also looked at whose sleep was disrupted the most. “We hypothesized and expected that people who were already living in warm climates would be better adapted to nighttime temperature increases,” says Kelton Minor, a PhD candidate at the University of Copenhagen’s Center for Social Data Science and the lead author of the study. “What we found was the exact opposite.” A 1-degree rise at night appears to affect residents of the world’s warmest climates more than twice as much as residents of the coldest regions, according to the analysis, which was based on data from 2015 to 2017.
They also found that sleep loss per degree of warming appeared to be greater among women, the elderly, and people in low-income countries. Although the study design didn’t allow for causal inferences as to why this is so, some conjecture can be made based on existing research: Women’s bodies usually cool down earlier in the evening to prepare for sleep than men’s, so women will face hotter, more disruptive temperatures when their sleep wave kicks in. Women also have higher levels of subcutaneous fat, which may slow the cooling process at night, making controlling body temperature in heat waves harder. And as we age, the body secretes less melatonin, which may explain why older people have even more difficulty regulating their body temperature when it’s too hot.
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Fans and air conditioners can help to remove heat from the body or cool a bedroom, but in lower-income countries most people do not have access to such devices. Apart from that, sleep researcher Blume has no single recipe for getting enough sleep on hot nights. “Anything that helps lower the body temperature would make sense from a sleep physiology perspective,” she says. Even something as simple as sleeping with a thin cover or without one at all, or taking a cooling hand and foot bath before bedtime, is useful—as long as the water is not too cold, because otherwise the body starts to compensate and produce heat, she says.
Removing electronic devices (which emit heat) from your room, keeping curtains, blinds, and windows shut during the day, and staying hydrated can all help too. “You just have to try things out. The main thing is to relax,” says Blume. But as you lie there sweltering, damp with sweat, that’s easier said than done.
Updated 7-16-2022 16:30 pm ET: A previous version of the article incorrectly stated that the study by Minor et al. estimated that people had lost 44 hours of sleep per year since 2010. The study actually found that people were losing that amount of sleep in 2010.