Climate graphic of the week: how sleep fares during climate change

Disrupted sleep from extremes in temperature due to climate change can be added to the long list of reasons for lost nights and poor productivity.

Temperature variations have long been anecdotally related to poor-quality sleep, but recent tracking of more than 7mn sleep records across 68 countries using physiological measures of sleep from fitness wristbands has confirmed the extent of the problem.

An original study in 2017 produced by quantitative social scientists from MIT and University of California, San Diego first attempted to shed some light on how much variations in temperature affect sleep.

That survey of 765,000 US respondents reported that increased night temperatures were linked to a higher number of nights of “insufficient sleep” in the self-reporting by participants. 

Each 1C increase in night temperature translated into the average person in the US having 0.03 more nights of insufficient sleep each month. Although this did not seem notable on an individual level, when extrapolated across the US population it translated to about 9mn additional nights of insufficient sleep per month, or about 110mn extra nights of insufficient sleep annually, the study concluded.

Taken together with the widely documented effect of sleep deprivation on productivity, these population-wide numbers illustrate the high economic cost that can be associated with sleep deprivation as a result of rising temperatures.

In parallel, poor-quality sleep is also linked to a range of adverse health outcomes, ranging from impaired attention and memory to heart disease and premature death. According to the World Health Organization, small seasonal temperature anomalies due to climate change on their own have wide ranging physiological impacts on human health and can worsen a range of chronic conditions such as cardiovascular, respiratory, and cerebrovascular disease.

The additional burden of sleep disturbance as a result of rising temperatures will probably exacerbate the existing public health issues resulting from climate change.

The lead academic in the sleep study, Nick Obradovich, suggests that some of the negative health outcomes related to climate change may be driven in part by the lack of sleep.

While the data indicates we will lose more sleep as the planet continues to warm, hot nights are already affecting some groups more than others. The same study showed that those on lower incomes experience almost four times more nights of insufficient sleep compared to wealthier individuals, possibly because of the costs associated with air conditioning.

Like many health issues, age is also an influence. Older people can have deficient thermoregulation, making their sleep cycles more vulnerable to anomalous temperatures. The study also found that people over 65 reported losing twice as much sleep per 1C of increased temperature compared to younger people.

But human memory is notoriously unreliable, particularly when it comes to the detail. This is something Obradovich, now a senior research scientist and principal investigator at the Max Planck Institute for Human Development, was concerned about.

“If you are asking people to remember how well they slept last night, or to report how they slept a few days ago, there is just a lot of opportunity to misremember,” he says.

As a result, Obradovich and his colleagues conducted the follow-up study in 2022 using fitness trackers that allowed them to replicate the original findings across a wider and more diverse population.

The extent of the diversity in this research area remains problematic, however. There is still little data from those in the lowest income group in countries that are the most vulnerable to the effects of extreme temperatures.

Warm nights can be particularly bad in cities, as urban areas typically have higher temperatures compared to more rural surroundings. A study from the School of City and Regional Planning at the Georgia Institute of Technology found that large US cities had been “warming at more than twice the rate of the planet as a whole”.

Hot temperatures at night also impact our cognitive abilities, either directly or through lack of sleep. In a separate study from the Harvard Department of Environmental Health, researchers found that higher night-time temperatures during sleep were associated with a decrease in performance in attention and arithmetic tests. Using statistical methods, this study further showed that sleep might be a factor in the causal mechanism between hot temperature exposures and cognitive function deficits. 

Temperatures have already risen at least 1.1C since pre-industrial times due to greenhouse gases resulting from human activity.

If carbon dioxide continues to rise unabated and warms the planet further, our strategic decision-making skills could be 50 per cent lower by 2100, according to another recent study by researchers from the University of Pennsylvania and the University of Colorado Boulder.

Decreasing sleep quality, and its associated health risks, might seem like a modern problem, but complaints of poor-quality sleep have been recorded at least as far back as the late 19th century. However, the estimates of an increase in temperatures of 2.7C by the end of the century will have an undoubted affect on sleep across the population.