By the end of this century, Americans could lose up to an entire day of sleep annually as rising temperatures turn summer nights into restless struggles with sheets and pillows, according to new research tracking 12 million nights of sleep across the United States.
The study, published in Environment International, reveals a striking divide: coastal residents from San Diego to Seattle face sleep losses nearly three times worse than the rest of the country. In August, people living in these marine climate zones could lose more than 10 minutes of sleep every single night when temperatures spike.
Researchers from the University of Southern California analyzed Fitbit data from over 14,000 adults, matching their sleep patterns against local temperature records from 2010 to 2022. The scale of the dataset allowed them to spot something previous studies missed: the inequality of who suffers most.
Renters lose significantly more sleep than homeowners. Hispanic individuals experience 30% greater sleep disruption than non-Hispanic populations. People earning under $35,000 annually face 36% more sleep loss than those making over $150,000. Women lose more sleep than men. The pattern holds for people with cardiovascular disease, obesity, and depression, all of whom prove more vulnerable to heat’s effects.
“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?”
Why the West Coast Suffers Most
The marine climate zone’s vulnerability likely stems from a simple fact: only 51% of households in these coastal areas use air conditioning, compared to 89% nationwide. Residents adapted to mild summers suddenly face heat waves without the infrastructure to cope.
The sleep losses concentrate in summer months, peaking between June and September. Heat disrupts sleep through multiple pathways. The body needs to cool down to trigger sleep onset, but elevated nighttime temperatures prevent this natural temperature drop. Heat also activates stress responses, causes dehydration, and reduces time spent in deep sleep and REM stages, the phases most critical for physical recovery and memory.
For every 10 degrees Celsius increase in nighttime temperature, people lost an average of 2.6 minutes of sleep. Daytime heat also mattered, though slightly less. Beyond duration, the researchers found that warmer nights degraded sleep quality: people took longer to fall asleep, woke more frequently, and spent less time in restorative sleep stages.
The Rental Housing Crisis Extends to Sleep
The disparities reveal how housing inequality translates directly into health inequality. Renters typically cannot install air conditioning, upgrade insulation, or modify their homes for heat resilience. Lower-income individuals often live in neighborhoods with less tree cover and more heat-absorbing concrete. Single people and widows showed 30% more sleep disruption than married couples, potentially because they cannot share cooling costs or have less control over housing decisions.
“This may seem like a small amount, but when it adds up across millions of people, the total impact is enormous.”
The projections assume different climate scenarios through 2099. Under high-emission pathways, West Coast residents could lose 24 hours of sleep per year, while those in colder regions might lose 8 to 12 hours. These estimates likely understate the true impact, since they do not account for other climate-related disruptions like wildfire smoke, hurricanes, or flooding that also interfere with sleep.
The researchers suggest policy responses tailored to vulnerable populations. West Coast cities could subsidize air conditioning for renters, expand urban forests, or mandate heat-resistant building designs. Strengthening building codes to require better insulation would help, particularly in rental housing where tenants have no authority to make improvements.
The team plans follow-up studies testing whether cooling interventions can protect sleep and, by extension, reduce heat-related health problems. Poor sleep increases risk for cardiovascular disease, mental health disorders, and cognitive decline, the same conditions that heat exposure worsens directly.
This August, if you find yourself awake at 2 a.m. in Portland or Sacramento, kicking off blankets and flipping pillows to find the cool side, you are experiencing a health impact of climate change that researchers are only beginning to quantify. The lost sleep adds up: night after night, summer after summer, year after year, until an entire day disappears.
Environment International: 10.1016/j.envint.2025.109942
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