New research on how warm nighttime temperatures disrupt sleep
Do you have trouble sleeping on hot summer nights? It’s a complaint I heard from many of my patients this summer, especially in California, where a late-summer heat wave brought triple-digit temperatures to many parts of the state.
Ambient temperatures—the temperature of the environment that surrounds you—can have a significant effect on how much and how well you sleep. A new, large-scale study of the effects of temperature on sleep has returned some really interesting information about how hot nighttime temperatures affect sleep—and about who is most likely to be affected.
What happens to body temperature when we sleep?
Before I talk about this new research, let’s look at the role of body temperature in sleep. A higher body temperature can interfere with your ability to sleep. What does body temperature have to do with sleep? As your body prepares itself for sleep, you experience a drop in core body temperature, which begins in the late afternoon and continues into the evening hours.
The body’s process of thermoregulation—its ability to maintain and adjust its core temperature—operates on a 24-hour circadian cycle, as does the sleep-wake cycle. Falling and rising body temperature over the 24-hour day is an important contributor to the body’s sleep-wake cycle: dropping body temperature helps you fall asleep and stay asleep at night, and rising temperature stimulates alertness in the morning.
In preparation for sleep at night, the body pushes heat to the extremities. Blood vessels on the skin become larger in order to release heat. These physiological changes work to lower your core body temperature. With that drop in body temperature comes feelings of drowsiness, and eventually, sleep itself. Body temperature stays low throughout the night before beginning to rise in the very early morning hours, helping prepare you to wake up and be active and alert.
How ambient temperature affects sleep
Our bodies are well designed to regulate our internal temperature. Sweating and shivering are two overt signs of the body making adjustments to its temperature, in response to our environments and other factors, such as illness, stress, and exertion. But body temperature is still strongly influenced by the temperature of our environment.
In fact, the temperature of our surrounding environment is one of the most important factors that influences sleep.
Hot temperatures make it harder for the body to shed heat and cool itself. Steamy and humid nights cause people to wake more often during the night, reducing sleep efficiency. Sleep efficiency is a measurement of sleep quality, based on the amount of time you spend in bed compared to the amount of time you spend actually sleeping. A solid sleep efficiency score is 85 percent—that means of the 100 percent of the time you spend in bed, 85 percent is spent sleeping. Really good sleepers cross the 90 percent threshold. On especially warm nights, your sleep efficiency is likely to be lower if you’re not taking steps to keep your surrounding environment cool.
Steamy temperatures also diminish time spent in slow-wave sleep and REM sleep. These sleep stages are when the body does critical work to rejuvenate and restore itself, from repairing cells and strengthening its immune system to processing memory emotions. Exposure to very warm and humid ambient temperatures can limit your body’s chance to do this important work.
And it doesn’t take a record-breaking heat wave to experience these negative effects on sleep. Research shows that even mild heat exposure can keep body temperatures higher, alter time spent in sleep stages, and make it harder to fall asleep and stay asleep.
This latest research focuses on the effects of above average temperatures on sleep. But it’s not only a too-warm or hot environment that interferes with nightly rest. Cold temperatures affect sleep as well—altering time spent in REM sleep and other sleep stages, keeping blood pressure elevated, and making it harder to fall asleep and stay asleep.
Unusually hot nights make us more sleepless
The recent study, conducted by a team of researchers from leading universities in the United States, is the largest ever conducted that looks at the relationship between sleep and ambient temperature.
Scientists used data from 750,000 sleep surveys conducted by the Centers for Disease Control between 2002-2011, and compared that information to historical nighttime temperature data, to determine the effects of environmental temperature on sleep quality. Scientists were particularly interested in understanding the effects of above-average nighttime temperatures on the quantity and quality of sleep.
What did they find?
There’s a strong link between atypically high nighttime temperatures and insufficient sleep. Researchers found that a 1C° deviation from average monthly nighttime temperatures creates an additional 3 nights of disrupted sleep for every 100 people. Extrapolated to the population of the U.S., that’s 9 million additional nights of poor sleep a month, and 110 million additional nights of sleep annually.
Sleep disruption from nighttime temperatures most severe in the summer. The magnitude of the effects of nighttime temperatures on sleep disruption is nearly three times as high during the summer, compared to all other seasons, according to their findings.
Income and age are major factors. Researchers looked whether income had a role to play in making people more vulnerable to the temperature-effects of sleep disruption. Using a $50,000 annual median income as the dividing line, they split the study population into two groups: lower and higher income.
They found in the lower-income group, the effect of nighttime temperatures on sleep disruption was more than three times as high than for people with higher incomes.
Why would nighttime temperatures affect lower income populations more significantly? People with lower incomes may have fewer resources to put toward keeping cool on unusually warm nights. Less likely to have air conditioning or other ways to control the temperature of their surroundings, people with lower incomes are more likely to experience sleeplessness and poor-quality sleep. Because sleep effects nearly every part of life, people with lower incomes may also be more vulnerable to the health and safety risks, and the performance issues, that arise from poor sleep.
Among elderly people, the effects of atypically warm nights were found to be more than two times higher, compared to the general population. As we age, the body becomes less adept at thermoregulation, which may make older adults more susceptible to the effects of ambient temperature on their sleep.
Scientific evidence shows that gender plays a role in the age-related changes to the body’s ability to regulate its own temperature. With age, women may experience more pronounced changes to their bodies’ ability to thermoregulate—and may have stronger disruptions to their sleep, including:
• Sleeping less overall
• Waking earlier
• Feeling less satisfied with sleep
Researchers found that among people who are both elderly and low-income, the magnitude of the effect of unusually warm nights on sleep is more than 10 times greater than for the general population. This new research shows this group of people is likely to be the most vulnerable to sleeplessness when nighttime temperatures are unusually high.
How to manage temperature to sleep better
By managing both your body temperature and the temperature of your surroundings, you can help yourself sleep more comfortably.
Pay attention to your indoor environment, both day and night
Of course, air conditioners and fans can help keep things cool throughout the day and night, when temperatures are uncomfortably high. So can keeping bedrooms and other rooms throughout the house dark, by drawing shades and closing curtains. Blackout curtains are a worthwhile investment: they offer significant light protection, they help keep rooms cooler in warm months, and help retain heat at night during cold months.
The best bedroom temperature for most people is between 65-75 degrees Fahrenheit.
Dress for the nighttime weather
Breathable fabrics such as cotton, linen, and silk are the best choices for sleep clothing. Avoid synthetics such as polyester, which don’t breathe well. On particularly warm nights, you may be most comfortable sleeping naked. When contending with cold temperatures, make sure you’re dressed to stay warm but also comfortable—don’t load up on clothes that might constrict you during sleep. And be sure to keep your feet warm—cold feet can make it harder to fall asleep.
Use the right bedding
The same natural fabrics that work for sleepwear are also good choices for bedding, including wool and down. Don’t opt for synthetic fabrics in your sheets and bedcovers, as these materials trap both heat and moisture. Invest in the highest-quality bedding you can afford—it will be the most comfortable, whatever the temperature. Cover yourself in bed with as little as possible on warm nights. Some people need a light sheet even on hot nights in order to relax and feel comfortable falling asleep, while others can sleep on a bottom-sheet only.
Don’t exercise too close to bedtime. Body temperature stays elevated for about four hours after you finish exercising. Exercise in the evening can impede and delay that important downward shift in body temp, and keep you awake. On hot nights, this may be a particular issue, so plan to exercise earlier when it’s unusually warm.
Temperature can sometimes be an overlooked factor in sleep. It’s important to manage the temperature of your environment on a daily (and nightly) basis, for the benefit of your rest. When the forecast calls for unusually high temperatures that stretch into the night, take additional steps to keep your sleeping environment cool and comfortable. You’ll sleep better and be more rested the next day.
Michael J. Breus, PhD, DABSM
The Sleep Doctor™