Heat means less sleep: study

Canadians could lose some 58 hours of sleep a year by the end of this century due to global heating, a new study suggests.

University of Copenhagen PhD candidate Kelton Minor published a study in One Earth May 20 on how rising temperatures are chipping away at human sleep.

“We spend nearly a third of our lives asleep, but still growing numbers in many countries do not sleep enough, even though sound and sufficient slumber is essential for human functioning,” Minor said in an email interview.

Researchers know relatively little about how outside temperatures affect sleep patterns globally, Minor said. He and his team used sleep-tracking wristbands in 68 nations to track the sleep patterns of about 48,000 adults from 2015 to 2017 and combined that data with temperature records and climate model projections.

The team found adults generally fell asleep later, rose earlier, and slept less during hot nights. People slept about 14 minutes less on very warm (over 30 C) nights, and were 3.5-per-cent more likely to get less than seven hours of sleep when temperatures were above 25 C than they were at 5 to 10 C.

Excess heat at night already costs the average person about 44 hours of sleep a year (about 11 sleep-short nights), the team estimated. Climate models suggest this will rise to 50 to 58 hours of lost sleep by the end of this century, or about 13 to 15 short-sleep nights.

Heat affects sleep through body temperature, Minor said. Our bodies shed heat at night and are most likely to fall asleep when they cool to a certain level. The hotter it is outside, the longer this takes, and the longer we take to fall asleep.

While previous research found both hot and cold weather made it tougher for people to sleep, Minor’s team found that heat had a greater impact, suggesting that people are better at adapting to cold than they are to heat.

“Unfortunately, we don’t find evidence that people are adapting well to warmer temperatures today,” Minor said.

The team found hot nights had an even worse effect on sleep in regions which are naturally warm and where one would expect people to be used to warm nights. They did not find evidence that people are able to make up for lost sleep with naps or extra sleep on other days.

Minor said this study provides the first planet-wide evidence that hot nights erode sleep, and do so by delaying when people fall asleep and advancing when they wake up. Previous studies have found sleep shortages make anger, accidents, and hypertension more likely, which could explain why we see more deaths and hospitalizations on hot days.

This impact will fall disproportionately on the old, poor, and female, Morin noted. The sleep loss per degree of warming was twice as large in seniors than it was in other age groups, three times larger in poor nations than rich ones, and significantly larger for women than men.

Morin said the study found cooler temperatures promoted sleep gain, which implies air conditioners could shield against sleep loss. Air conditioning could also create greenhouse-gas emissions and worsen warming, however, and might not be affordable in the nations most affected by warming. Policy makers need to ensure equal access to cooling technologies in a warming world.

The Edmonton region is projected to have about six to eight more days of +30 C weather a year by 2050 based on current greenhouse-gas emission trends, the Climate Atlas of Canada reports.

The study can be found at bit.ly/3LAH1qH.