Getting less than seven hours of regular sleep a night, the minimum standard for adults, has also been linked to heart problems, obesity and type 2 diabetes, among other conditions. “People try to sleep in during the week and then catch up on the weekend, but you never fully offset the health and cognitive benefits of getting enough sleep throughout the week,” says Miller.
And with climate change, warm, sleepless nights are now being experienced by many people around the world. Compared to the beginning of the 21st century, nighttime temperatures are higher today, meaning that, worldwide, each person is losing an average of 44 hours of sleep per year compared to what they were getting in 2010. This also means that, on average, adults are experiencing an extra 11 nights each year when they sleep less than the seven hours they need.
As air temperatures continue to rise, people could be leaking even more. A recent study linked the sleep trackers of more than 47,000 people in 68 countries to local weather data and predicted that individuals could lose 50 hours of sleep per year by the end of the century, compared to 2010. The six additional hours lost are spread over the years between now and it might not seem like much at the time, but it would result in about 13 extra short nights of sleep, which is hardly welcome.
The study researchers also looked at whose sleep was most disrupted. “We hypothesized and expected that people who already live in warm climates would be better adapted to the nighttime rise in temperature,” says Kelton Minor, a PhD student at the University of Copenhagen’s Center for Social Data Science and lead author of the study. “What we found was just the opposite.” An increase of 1 degree 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 an analysis based on data from 2015 to 2017.
They also found that sleep loss by degree of warming was greater among women, the elderly and people in low-income countries. Although the study design did not allow for causal inferences about why this is the case, some assumptions can be made based on existing research: Women’s bodies tend to cool down earlier in the evening in preparation for sleep than men’s, so women will face warmer, more disruptive temperature when their sleep wave starts. Women also have higher levels of subcutaneous fat, which can slow the cooling process at night, making it harder to control body temperature during heat waves. 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.
Fans and air conditioners can help remove heat from the body or cool the bedroom, but in lower-income countries most people do not have access to such devices. Besides, sleep researcher Blume doesn’t have a single recipe for getting enough sleep on hot nights. “Anything that helps lower body temperature would make sense from a sleep physiology perspective,” she says. Even something as simple as sleeping with or without a thin blanket, or a cooling bath for your hands and feet before bed, is helpful—as long as the water isn’t too cold, or the body starts to compensate and produce heat, she says.
Removing electronic devices (which emit heat) from your room, keeping curtains, blinds and windows closed during the day and staying hydrated can also help. “You just have to try things. The main thing is to relax,” says Blume. But when you’re lying burnt, drenched in sweat, that’s easier said than done.