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.
