Baniassadi and his fellow researchers also found that temperature starts to play “an important role” once it goes above 77 or below 68, although they mostly were interested “in what happens when it gets hotter,” he said. They found a 5-to-10 percent drop in sleep efficiency when indoor temperatures rose from 77 to 86 degrees, he said.
“The consensus among most scientists is that anything higher than 75 or 80 is bad for sleep and potentially detrimental to health,” Baniassadi said.
Heat affects sleep because the body signals the brain that it’s time to fall asleep by lowering its core temperature. Sleeping in a hot room makes this difficult.
The findings, published recently in the journal Science of the Total Environment, underscore the conclusions of sleep and climate experts that hot nights disrupt sleep and can lead to harmful health consequences.
The study was conducted in the Boston area between October 2021 and April 2023 and tracked 50 participants, 65 and over, each for a 12-month period.
The researchers installed environmental sensors in the bedrooms of the participants, which tracked nightly temperatures and asked sleepers to wear a special monitor — a finger ring — linked to a smartphone.
The rings measured how long people slept, the ratio between sleep and the time they spent in bed, and movements such as tossing and turning. The monitors also recorded respiration and heart rate, and body temperature deviations from one night to another.
The subjects came from different living conditions, including some in subsidized housing and some who had no access to air conditioning. Some were healthy, while others had health issues, Baniassadi said.
“We selected older adults because they typically experience poor sleep more than younger populations, their physiology is more sensitive to temperature changes, and they suffer the most in heat waves,” he said.
The researchers found that sleep was “efficient and restful” when the indoor night temperature was between 20 and 25 degrees Celsius (68 to 77 degrees Fahrenheit), and sleep efficiency dropped when the temperature increased from 25 to 30 degrees Celsius (77 to 86 degrees Fahrenheit).
These results are not surprising — a growing body of evidence links bad sleep to high heat — but the work is a departure from other research in that it focuses on indoor air and on participants while they are living in their homes and sleeping in their own beds.
Typical sleep studies take place in a lab, where scientists measure sleep outcomes under varied and controlled temperatures or, for example, in other studies, that correlate self-reported sleep behaviors with recorded temperature data.
“We were trying to conduct the study as close to normal living conditions as possible,” Baniassadi said.
The new research is a useful contribution to the field because it looks at “indoor temperatures,” but is limited by the small number of participants, said Nick Obradovich, chief scientist for environmental mental health at the Laureate Institute for Brain Research and author of several studies connecting high nighttime temperatures to impaired sleep. This, he said, made it “quite possible that their study findings do not hold more broadly.”
“One of the strengths of observational work is typically that one can get much larger samples than we’d be able to get into a lab,” added Obradovich, who was not involved in the study. “That strength doesn’t really apply with this particular study, given their small sample size.”
Baniassadi acknowledged the limitation but called the decision “a trade-off” saying larger numbers would have prompted a shorter study time and “we wanted to monitor them for a longer period of time.”
Poor sleep is linked with worse health outcomes
The link between higher temperatures and poor sleep is important because, according to the Centers for Disease Control and Prevention, insufficient and unrestful sleep, less than seven hours a night, raises the risk of, among other things, Type 2 diabetes, obesity, heart disease, depression, motor vehicle accidents and deficient work performance. One-third of U.S. adults report getting less than the recommended amount of sleep, the CDC says.
Nighttime temperatures are rising faster than those during the day, in large part because of our warming planet, and this may lead to worse sleep outcomes for vulnerable populations such as older people and those without air conditioning, experts say.
“We see the impacts of climate change almost daily in persistent heat waves, devastating wildfires, dangerous hurricanes, and floods, but what often receives less attention is the impacts on human health,” said climatologist Michael Mann, director of the University of Pennsylvania’s center for science, sustainability and the media, and author of the forthcoming book, “Our Fragile Moment.”
“Among the more pernicious of these is the steady rise in nighttime temperatures, which contributes to impaired quality of sleep, particularly among the vulnerable,” said Mann, who was not involved in the study. “These include the elderly and populations that have no refuge from the unrelenting heat, such as those in communities where air conditioning is a luxury people cannot afford.”
Tips to regulate temperature for better sleep
Most sleep specialists recommend people sleep in a “cool, dark and quiet environment,” typically ranging from about 65 to 75 degrees, said Ronald Chervin, the division chief of the sleep disorders centers and professor of sleep medicine at the University of Michigan, who was not involved in the study. “There also are plenty of people who will tell you they sleep better at less than 68,” he said.
Baniassadi suggested the following strategies for those who want to minimize the effects of high heat on sleep for themselves and others. He cautions though that “each person may have their own optimal temperature,” he said. “Observe yourself and try to understand what temperature is good for you. Every person is different. We can’t say, for example, that 72 degrees is good for everyone. It’s something that people need to figure out on their own.”
- During warmer nights, stay hydrated.
- Take a shower before bed.
- Wear light sleepwear.
- If you have the means to manage the temperature in your bedroom, adjust the thermostat.
- Open a window.
- Make sure your air conditioner is working properly, especially during a heat wave.
- Be mindful of others in your community. Check on older family members and neighbors when it’s hot.
On a societal level, “we also need to cool down our cities” by investing in heat-resilient buildings to reduce reliance on air conditioning, he said.
“Most cities around the world are getting warmer at night, mostly due to climate change, and that’s having a bad effect on sleep,” Baniassadi said. “Sleep influences everything. It’s good for your brain and the rest of the body. When sleep deteriorates, everything else follows.”
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