Rising sea levels, more extreme weather, droughts and famine, melting heatwaves — these are just a few of the grim effects expected due to man-made climate change, many of which are already being felt. It’s no wonder people are losing sleep over it. But it may not just be the anxiety at play here.

In a new study, researchers at the University of Copenhagen found that by the end of this century, people worldwide could literally lose between 50 and 58 hours of sleep per year due to hotter nights that are suboptimal for human sleep. Since sleep is fundamentally important to our health and mental wellbeing, poor sleep as a result of global warming could have spillover effects in other areas of our lives — and like it’s the case with these things, the impact will be highly unequal.

“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 are essential for human functioning. Yet before our study, relatively little was known about how outside ambient temperatures – which are warming faster at night in most regions – impact human sleep outcomes globally,” first author Kelton Minor of the University of Copenhagen told ZME Science.

“We found that warmer than average nights harm human sleep globally and unequally: people sleep less and the probability of having a short night of sleep steeply increases as temperatures become hotter. Importantly, we found that this hidden human cost of heat is not distributed equally: the sleep loss per degree of warming is twice as large among the elderly compared to younger or middle-aged adults, three times larger for lower-income versus high-income countries, and significantly larger for females than males,” he added.

Too hot for comfort

You might want to check your thermostat before bedtime after reading all of this. While opinions vary and every person is different, the optimal ambient temperature range for optimal sleep sits between 15-22℃ (60-72℉). Anything colder and hotter can cause your sleep to suffer due to increased wakefulness and decreased rapid eye movement (REM) and slow wave sleep, though heat is a much bigger problem than the cold.

The core body temperature follows a circadian rhythm, meaning it rises and falls across a 24-hour cycle. When the body temperature drops, this is a signal for our bodies to prepare for sleep — this is a fundamental characteristic that is common across most mammals. During the night, for instance, your core body temperature should drop by as much as 1°C, but if it’s hot outside, the body will have difficulties transferring heat to its surroundings.

Conversely, increases in core temperature during sleep promote waking. Around two hours before you’re supposed to wake up, the core body temperature begins to rise and continues to do so until it peaks around early evening.

Although it’s an established fact that hot weather negatively affects the quality of sleep, it’s unclear how fluctuations in temperature impact sleep outcomes across a variety of global climates. Both studies that use self-reported data or monitor sleep in a controlled environment have their limitations since in the real world people will actively seek to control their thermal sleeping environments.

A planet-scale natural experiment

To shed more light on the matter, the team decided to conduct what they call a “planet-scale natural experiment”, which linked over 10 billion sleep quality measurements from smart wristbands used by more than 47,000 people across 68 countries with local weather and climate data. Since the participants went about their lives as they normally would, their data reflect more natural sleep patterns than in controlled studies, such as those that monitor people in a lab.

“Early sleep laboratory studies featuring human, as well as animal subjects, found that subjects had an inverse “U”-shaped response to temperature in highly controlled settings. Invoking the Goldilocks metaphor, subjects slept worse when room temperatures were either too cold or too warm. However, a key limitation of this early research was that it constrained natural human adaptation. In the real world, people may modify their thermal sleeping environment: when possible we select for comfort. But studying sleep behavior in the real world is notoriously complex. One of the advances in our global study is that we were able to statistically control for location-specific seasonal changes, including general changes in leisure behavior across summer months,” said Minor.

According to this data, during very warm nights with ambient temperature over 30℃ (86℉), people had 14 minutes less of sleep on average per night, and the likelihood of getting less than 7 hours of sleep increased. Your body needs at least seven hours of sleep, just as it needs air and food to function at its best.

“Prior sleep research from both the sleep lab and real world has robustly demonstrated that insufficient sleep increases the risk of many of the same negative physiological, behavioral, social, and economic outcomes previously observed to increase during hot days. Thus, by elevating the probability of short sleep, higher ambient temperatures may predispose susceptible segments of society to worsened mood, adverse cardiovascular outcomes, diminished cognitive performance, greater risk of injury, and compromised immune system functioning. However, future research is needed to examine the full downstream consequences of temperature-driven sleep erosion on human well-being and performance,” said Minor.

Minor and colleagues found that people seem to be able to adapt far better to colder ambient temperatures than hotter conditions. This makes sense since it is generally more accessible to wear warmer clothes or use a thicker blanket than to turn on expensive air conditioning.

The impact of hot nights on sleep could be three times larger in poor countries than in rich countries

Although the researchers’ data did not inform them which participants used air conditioning, it is reasonable to assume that those living in developed countries will be better placed to mitigate the impact of climate change on sleep than those living in the world’s poorest and hottest regions. The evidence so far suggests that the impacts of temperature on sleep are up to three times larger in lower middle-income countries compared to high-income countries.

Actually, the researchers were quite surprised to find that people living in hotter climates were not particularly adapted to nighttime heat. And although the worst loss of sleep quality is projected for the end of the century, high nighttime temperatures may already be eroding sleep today compared to a few decades ago, and these effects are distributed unequally across the world.

“By coupling our sleep response function with local climate projections for the US and the other 67 countries in our study, we observed that annual sleep erosion is projected to grow faster in the south and southwest of the US as well as in more equatorial regions globally. Importantly, the projected disparities between the world’s coldest and warmest climate regions scale with the level of future greenhouse gas concentrations,” said Minor.

According to a 2020 study from the University of Exeter, greater nighttime warming is more common than greater daytime warming worldwide. In over half of the world, nighttime warming is about 0.25°C higher than daytime warming. This presents tough challenges that may require huge costs to mitigate, however, these costs and the benefits of these special measures are largely unknown for now.

“Our results indicate that sleep – an essential restorative process integral for human health, cognition, and performance – may be degraded by warmer temperatures. In order to make informed climate policy decisions moving forward, we need to better account for the full spectrum of plausible future climate impacts extending from today’s societal greenhouse gas emissions choices, including impacts to our daily activities that we might otherwise take for granted,” Minor concluded.

The findings appeared in the journal One Earth.