Higher nighttime temperatures are associated with shorter and more disrupted sleep, particularly for individuals with chronic health conditions, lower socioeconomic status, or those living on the West Coast, according to a recent study by researchers at the University of Southern California (USC). The findings were published in the journal Environment International.
The study estimates that by 2099, people could lose up to 24 hours of sleep each year due to increased heat. Researchers note that warm weather can disturb sleep by preventing the body from cooling down, triggering stress responses, and reducing time spent in deep and REM sleep. Poor sleep is linked to heightened risks for heart and respiratory issues as well as mental health concerns.
"We already know that when there are extreme heat events, more people die from cardiovascular disease and pulmonary disease. What will this mean for population health as global temperatures continue to rise?" said Jiawen Liao, PhD, a postdoctoral research associate at the Keck School of Medicine of USC and first author of the study.
Previous studies have shown a connection between higher temperatures and sleep problems but lacked detailed demographic and health data. This new research addresses those gaps using information from 14,232 U.S. adults who participated in the All of Us Research Program—a longitudinal study collecting survey responses, health records, wearable device data such as FitBit information, and other sources.
"This work is an important step toward understanding how sleep is affected by environmental stressors like heat, which can increase the risk of disease and even death," Liao said. "If we can help people sleep better, we may be able to reduce illness and save lives."
Researchers analyzed over 12 million nights of recorded sleep data to assess duration and ease of falling asleep. They also reviewed 8 million nights focusing on specific sleep stages and interruptions. Location-based meteorological data was used to examine links between temperature changes and sleep patterns.
Results showed that a 10-degree Celsius increase in daytime temperature led to an average loss of 2.19 minutes of sleep per night; a similar nighttime increase resulted in a loss of 2.63 minutes per night. These effects were more pronounced among females, Hispanic individuals, those with chronic diseases or lower socioeconomic status.
"This may seem like a small amount, but when it adds up across millions of people, the total impact is enormous," Liao said.
The impact varied by season—most notably from June through September—and by region; residents on the West Coast lost nearly three times as much sleep compared to other regions. Projections suggest U.S. adults could lose between 8.5 and 24 hours of annual sleep by century’s end depending on location.
In addition to shorter durations overall, rising temperatures correlated with more frequent disruptions during the night and increased time spent awake while in bed.
The study highlights greater vulnerability among certain populations; targeting interventions such as improved access to air conditioning or urban green spaces may be especially effective for these groups. Strengthening building codes related to insulation or ventilation could also help protect public health in affected areas.
Liao indicated future research will focus on whether interventions—including indoor cooling measures or programs promoting good sleep hygiene—can mitigate negative impacts from heat exposure on both sleep quality and broader health outcomes.
