The study of millions of hours of sleep of thousands of people from almost all over the world confirms what experience says: the hotter it is at night, the worse you sleep. But the details of this recently published work were not so obvious: people in the poorest countries have it the worst, along with the elderly and women. In addition, the authors of the analysis, carried out over three years, have crossed their data with climate projections and calculate that, by the end of the century, we will sleep more than 50 hours less per year due to the increase in hot nights.
Research carried out under controlled conditions has shown that the optimal temperature for sleeping is around 17º-18º. From there, the quality and quantity of sleep begins to suffer. Excess environmental heat compromises the thermoregulation of the body’s central temperature (trunk and brain) through the peripheral, which uses blood circulation to the extremities as a cooling mechanism. This has a wide range of consequences that go beyond drowsiness the next day. But it is not easy to study the real impact of climatic factors on sleep.
A group of researchers from Danish and German universities has managed to get almost 50,000 people from 68 countries who use wristbands that record activity (two models from a Japanese manufacturer) to participate in a project that wanted to cross-reference the data recorded by the device with the time it takes. did instead. The bracelet has an accelerometer with which you can infer when you fall asleep and when you wake up. This information was related to a wide range of weather data: temperature each night, maximum and minimum each day, thermal amplitude between the two, whether that day was cloudy, windy or the relative humidity of the air. The results of so much data crossing have just been published in the scientific journal One Earth.
“The heat delays the moment in which people fall asleep and advances when they wake up”
Kelton Minor, University of Copenhagen
The researcher at the University of Copenhagen (Denmark) and main author of the study Kelton Minor highlights that his work represents “the first evidence on a planetary scale that temperatures above the average erode the sleep of humans”. This erosion occurs mainly because “heat delays the moment in which people fall asleep and advances when they wake up,” he adds. So the total duration is shortened. The humans in the sample sleep around 7.1 hours. Half of the older ones don’t even get to that and the middle-aged ones have longer vigils on weekdays than on weekends. Globally, Asians sleep less than Europeans. The Japanese, for example, spend half an hour less than the average Italian or Spanish.
When the temperature factor is introduced, the study reveals data beyond the obvious impact of heat on sleep quality. Thus, on warmer nights compared to the average, its duration is reduced by just over 14 minutes on average. “Compared to the global nighttime average (nights in the 5-10C temperature range), we see that sweltering nights above 25C increase the likelihood of people getting little sleep by 3.5 percentage points,” he writes. Minor in a mail. “We estimate that roughly a single hot night in a city with a population equivalent of 100,000 adults would cause about 3,500 additional adults to have a worse night than nights when the temperature approached the global nighttime average,” details.
The work confirms something already studied: older people sleep worse when it’s hot, rather much worse. The effect of a one-grade increase is more than double in those over 65. He also notes that women tend to be more affected by excessive temperature than men. Although they hardly have data from Africa (only from Morocco and South Africa) and a few more from Latin America, the authors of the study observe that the negative impact is more pronounced in countries with medium and low levels of development than in the most advanced ones. It is likely that the air conditioning has its impact here, but they had no way of confirming it.
In the last part of the work, the scientists project their data into two climate scenarios: one in which emissions are controlled and another in which they continue to increase. Both situations will witness an increased number of nights with high lows. “Under a moderate climate warming scenario (where humans manage to stabilize greenhouse gas concentrations) and without further adaptation, we estimate that by 2099 each person may lose an average of 50 hours of sleep per year due to suboptimal nighttime temperatures.” Minor says. And if nothing is done, the figure could approach 60 hours.
“The problem with climate change and heat waves like the current one is that the body cannot cool down and the warmer the brain is, the worse the sleep will be”
Juan Antonio Madrid, researcher at the chronobiology laboratory at the University of Murcia
For Juan Antonio Madrid, a researcher at the chronobiology laboratory of the University of Murcia, “this work is brutal due to the enormous amount of data it handles”. Madrid has spent years studying circadian rhythms and, in particular, the physiology of sleep and the external agents that disturb it. “The problem with climate change and heat waves like the current one is that the body can’t cool down and the warmer the brain, the worse the sleep,” he adds. To sleep well, the brain has to lower its internal temperature. To do this, the body heats the feet and hands, through which the body eliminates excess heat. “It’s basic physics. If there is no thermal difference between the body and the environment, there is no exchange or drop in internal temperature”, comments Madrid.
The main problem that Madrid sees in this research is that it starts from an assumption that is not always fulfilled. “The bracelets measure sleep by the absence of movement, but being still does not imply being asleep.” In their laboratory they developed the kronowise, a smartwatch-shaped device that records 15 types of data, from light exposure to how much and how the body moves. One of its keys is that it takes the temperature on the wrist, so it records the increase in temperature in the extremities prior to and necessary for falling asleep.
Dr. Javier Puertas, neurophysiologist and vice-president of the Spanish Sleep Society (SES), recalls the effects of getting little sleep: “Sensation of physical fatigue, lack of concentration, irritability…” Puertas highlights an aspect highlighted by the authors of the research : the ability to adapt is reduced. “We have verified it with noise. People who go to live in a neighborhood near the airports end up ignoring the noise, but their cardiac activity does not, which has not adapted”. The same thing seems to happen with heat. The bracelets recorded that people slept just as poorly on hot late-summer nights as they did early. “We never fully adapt optimally when there are sleep disruptors like noise or heat,” recalls Puertas.
In the advice section, the vice president of the SES does not doubt it, a better fan (preferably a ceiling fan) than an air conditioner: “It is a gift from tropical countries. The fans do not lower the temperature, but they cause the air to circulate, which facilitates the internal mechanism to reduce the temperature [corporal] central”.
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