August 11, 2022

I, Science

The science magazine of Imperial College

By Alex Häuser 

27th July 2022      

The air is stuffy in the hostel at Rishi Valley School in India’s Andhra Pradesh State. Vaish is tossing and turning in her small bed. She knows it is going to be a rough night. The old school building provides hardly enough insulation, let alone any air conditioning, to safeguard against the oppressive heat outside. She fears she will wake up in yet another pool of sweat, depriving her and her schoolmates of what they need most to study for their final exams – good sleep.

Vaish, now a university student in the UK, remembers that during the unprecedented heatwave in India in April and May 2017, the daytime heat was gruelling enough, but it was the nights that were truly unbearable. Like many others, she has experienced how drastically global rising temperatures impact sleep. “We spend nearly a third of our lives asleep, yet growing numbers in many populations do not get enough of it”, says Kelton Minor, the lead researcher of a new study linking sleep loss – and by extension, all problems that come with it – to climate change.

Minor and his colleagues from the University of Copenhagen are interested in how outside ambient temperatures affect our sleep behaviour indoors. In their study, which was published in the journal One Earth, they looked at this across multiple climate zones in over 68 countries using wristband-based data from 50,000 people. The results show that people slept most when outside temperatures were below 10°C. Above a critical threshold of 25°C, the chances of sleeping less than seven hours ramped up, and when outdoor temperatures passed 30°C, people lost an average of around 15 minutes per night. 

That may not sound like a lot, but years of research have revealed that even slight sleep deprivation can considerably slow one’s ability to learn, intensify mood disorders and increase the risk of heart disease. For Minor this has become a problem at the population level: “If you look at the recent heatwaves […] in India or Pakistan you have over a billion people who have been exposed to extreme temperatures that, we expect, would erode substantial sleep, not just for a single day but actually for over a month”. 

Most of the continuous sleep loss appears to be attributable to a delay in falling asleep. Vaish remembers this all too well. “Every night, we would shower five minutes before going to bed and hope the breeze we get from walking from out of the shower would cool us down enough to fall asleep”, she recounts. While having trouble to fall asleep was one issue in her final-year-of-school routine, possibly more concerning was that her body did not seem to adjust, even after having lived through months of hot-night exposure. 

Vaish’s experiences are consistent with the findings that people do not sleep better by the end of a prolonged time of heat. “People are not adapting well to hot climates when it comes to sleep as of yet”, Minor and his colleagues conclude. The researchers also thought that those who live in warmer climates might have historical adaptations to deal with higher sleeping temperatures. However, in the warmest climates, the detrimental effect per degree of warming was more than twice as significant on sleep as in the coldest climates, going against an environmental adaptation notion. 

One piece of evidence regarding adaptation points to socioeconomics. Residents of lower- and middle-income countries seem to suffer significantly more disrupted sleep time than those from high-income ones, suggesting that appliances like air conditioning help buffer the night-time heat impact on human sleep. Vaish is thinking of her parents living in Chennai, a five-hour drive away from her old school. “We’re lucky enough to have air conditioning at home, but […] I can’t imagine how bad it is for people who don’t have these resources.” 

The case of sleep loss suggests that responding to global warming requires immense short-term personal, and economic costs. Minor is convinced that his research exposes the need to account for a larger spectrum of climate change effects extending from today’s greenhouse gas emissions choices to daily human behaviour. “It helps better understand many of these more pervasive human impacts which have largely flown under the radar of the climate impacts research community”, he says. “As opposed to being rare events, we are talking about population-level exposures and impacts so they should of concern to policymakers.”