A window of covered with blinds at nighttime, showing shadows of plants outside the roomShare on Pinterest
Stefan Köhler/EyeEm/Getty Images
  • Researchers investigated the effects of temperature on sleep quality via wearable sleep-tracking devices.
  • They found that higher nighttime temperatures are linked to poorer sleep quality globally.
  • The study concluded that climate change is projected to erode sleep and widen global inequalities.

Reduced sleep quality affects human mental health and cognitive functioning.

Some retrospective, self-report studies suggest that sleep quality reduces during warm weather. However, they may lack reliability due to their basis on memory instead of objective measures.

As a result, whether outside temperatures affect sleep quality remains unknown.

Recently, researchers analyzed a global sample of sleep data from sleep-tracking wristbands.

They found that increased outdoor temperatures are linked to lower sleep duration.

“Studies from multiple disciplines have repeatedly shown that poor sleep is implicated in a range of negative health outcomes, from reduced immune function to worsened cardiovascular outcomes to poorer mental health,” said Marshall Burke, Ph.D., Associate Professor of Earth System Science at Stanford University, who’s not involved in the study.

“Poor sleep also erodes performance at work and at school. The fact that temperature effects are so widespread and that hot nighttime temperatures will become increasingly common in coming decades, make these findings very important,” Prof. Burke told Medical News Today.

The study was published in One Earth.

The researchers examined 10 billion sleep observations for their study, comprising over 7 million repeated daily sleep records from 47,628 adults across 68 countries on every continent, excluding Antarctica. These observations included nighttime sleep duration and sleep timing: sleep onset, midsleep, and offset.

The researchers then compared this data with geolocated meteorological and climate data.

They found that increases in nighttime temperature reduce sleep duration regardless of location and that effects intensify as temperature increases.

They noted that the probability of sleeping less than 7 hours increases gradually up to 10°C, and when the temperatures exceed 10°C, the chance of reduced sleep increases at an elevated rate.

Nighttime temperatures higher than 25°C were linked to 14 minutes less sleep than those sleeping at temperatures below 10°C.

Certain demographics were more affected than others. A one degree Celcius increase in minimum temperature affected the elderly twice as much as other groups.

Those living in poorer countries were almost three times more affected than those in wealthier countries, and women were significantly more affected than men.

They further found that people do not adapt to sleeping in warmer temperatures meaning that sleep quality is generally poorer in warmer climates than in cooler ones.

How does outdoor temperature affect sleep quality? Experts say it depends on several factors.

“High temperatures can increase arousal and decrease slow-wave sleep (“deep sleep”), which is the physically restorative stage of sleep,” said Prof. Tony Capon, Director of the Monash Sustainable Development Institute, not involved in the study.

Nick Franks FRS, Professor of Biophysics and Anaesthetics at Imperial College London, also not involved in the study, told MNT that there is a well-established link between body temperature and sleep.

Prof. Franks said causal links might be “tricky” to find because there are many possible variables.

Nevertheless, the researchers noted that heightened effects among older adults might be due to a weakened thermoregulatory response to environmental temperatures creating a higher sensitivity to rising nighttime temperatures.

Slight sex differences between men and women, they wrote, may come as women tend to have thicker subcutaneous fat than men, potentially impairing nocturnal heat loss.

Moreover, females’ core body temperatures decrease earlier in the evening than males, exposing women to higher environmental temperatures at sleep onset.

“Sleep deprivation can impair judgment and increase the risk of injury while driving and operating other machinery. Chronic sleep deprivation increases the risk of other health problems, including obesity, diabetes, high blood pressure, and depression,” noted Dr. Capon.

Dr. Alison Hwong, MD, Ph.D., Fellow at the University of California San Francisco Weill Institute for Neurosciences, not involved in the study, told MNT:

“In the long-term, reduced sleep time may contribute to an elevated stress response, interfere with memory consolidation, and affect the immune system. Adequate sleep is important for healthy development, repair and restoration.”

“As a psychiatrist, I am concerned about the adverse mental health effects of sleep deprivation. For example, sleep deprivation can trigger manic episodes for people with bipolar disorder and worsen mood for people with depression.,” she added.

“Moreover, antipsychotic medications can interfere with thermoregulation, so people who use these medications may have more trouble with sleep in warming temperatures,” she explained.

“There will need to be prevention, monitoring, and outreach to vulnerable groups, such as children, the elderly, those who are unhoused, and people with serious mental illness,” she continued.

The researchers conclude that their findings carry significant implications for adaptation planning, policy, and research.

When asked about limitations to the findings, Ed Harding Ph.D., Postdoctoral Neuroscientist at the Wellcome-MRC Institute of Metabolic Science, University of Cambridge, also not involved in the study, told MNT:

“We should bear in mind that people who chose to wear sleep-trackers are likely to be more affluent than is captured by per country income data used in this study, and have greater access to adaptations such as air conditioning, so the effect may well be underestimated.”

Dr. Burke added: “It would be great if future studies were able to somehow collect more data in many lower-income regions of the world [as] Africa has nearly no coverage in their data.”

“It would also be great to collect more data on sleep quality instead of just quantity, as eroded sleep quality could be an alternate mechanism through which higher nighttime temperatures could affect health and economic outcomes.”