A new study led by the University of Southern California shows that as climate warming causes outdoor temperatures to continue to rise, nighttime high temperatures not only make it difficult to fall asleep, but also systematically erode sleep duration and quality, with particularly obvious effects on patients with chronic diseases, low-income people, and residents of the West Coast of the United States.The research team pointed out that when the temperature remains high at night, it is more difficult for the human body to cool down to enter the deep sleep stage. Sleep rhythms are disrupted, the risk of dehydration increases, and the sympathetic nerves remain in a state of "alertness." These all lay hidden dangers for cardiovascular diseases, mental health problems, and cognitive function decline.
The study, a collaboration between the University of Southern California, Harvard Medical School and Brigham and Women's Hospital, used wearable devices and meteorological big data to analyze the long-term sleep records of 14,232 American adults over a 10-year period. Participants provided data on sleep duration, difficulty falling asleep, sleep stages and interruptions through digital wearable devices such as FitBit, covering a total of approximately 12 million nights of sleep duration and sleep status, as well as 8 million nights of sleep structure and continuity records, and matched them one by one with refined meteorological grid and geographical location data to evaluate the specific impact of temperature changes on sleep.
The results showed that for every 10 degrees Celsius increase in temperature during the day or night, the average sleep time of participants was shortened by about 2 to 3 minutes, and high temperatures at night were significantly associated with reduced sleep efficiency, delayed sleep onset, and longer nighttime awakenings. Among different groups of people, adults aged 40 to 50 are most affected. Every 10 degrees Celsius increase in nighttime temperature reduces their sleep duration by about 2.76 minutes. Women are also more susceptible to the effects of heat than men, sleeping an average of 2.65 minutes less, a gap of nearly 23% more than men. Research leader Liao Jiawen said that these numbers may seem small, but when magnified to a population of millions, the public health impact is "extremely considerable."
The impact of temperature on sleep also shows obvious time and regional differences: the warm season from June to September every year becomes the most serious period of sleep loss, and people lose more sleep during this period than in other months of the year. From a regional perspective, residents on the West Coast of the United States are most affected by hot nights, with sleep loss almost three times that of other regions. People in oceanic climate zones also experience greater stress, with related effects nearly twice that of other regions. The research team predicts that by the end of this century, compared with the period from 1995 to 2014, U.S. adults may lose approximately 8.5 to 24 additional hours of sleep per year due to rising nighttime temperatures, and the average annual sleep loss for residents of oceanic climate zones may increase to nearly a full day and night.
The researchers emphasized that the threat of high temperature to sleep is not evenly distributed, and some vulnerable groups are at significantly higher risks. Therefore, public policies and intervention measures should provide more precise "targeted support" to these groups. The team's next step will be to evaluate the actual effects of various mitigation options, including systematic sleep hygiene education projects, reducing urban ambient temperatures through "green roofs", and improving indoor cooling conditions, to explore whether these interventions can not only improve nighttime comfort, but also substantially reduce the burden of sleep-related diseases and mortality risks. Relevant research has been published in the journal "Environment International", with an abstract and interpretation released by the Keck School of Medicine of the University of Southern California.