Most of the United States is just cooling down after a difficult, stifling, sleep-depriving heatwave. About two-thirds of the country was affected by this latest heatwave, which saw temperatures near and above 100 degrees Fahrenheit, and heat indexes even higher. (The heat index measures how the body actually feels, taking into account both heat and humidity.)
This wasn’t the first heatwave of the summer. And it’s not likely to be the last, especially since we’re headed into what are typically the hottest days of the year. As many of us have just been reminded, hot temperatures and high humidity make it a lot harder to sleep. And when you can’t rest comfortably and sleep soundly, that makes everything else you do harder. Let’s take a look at how the body handles sleep and heat, and how you can set yourself up to sleep better when temperatures soar.
How heat hurts sleep
Sleep is highly dependent on temperature. Internal body temperatures play a critical role in sleep onset and sleep maintenance—those are scientific terms for our ability to fall asleep and stay asleep. Environmental temperatures—the ambient temperatures in our homes and our surroundings—also affect sleep directly, by interfering with the body’s ability to cool itself down.
Like most of the body’s physiological processes, thermoregulation—the body’s ability to maintain and adjust its core internal temperature—operates according to circadian rhythms, the biological rhythms that drive most of our daily functioning, including sleep-wake cycles. Temperatures fluctuate throughout the day, along with changes to energy levels, and other biological functions such as digestion, metabolism, and immune activity. As we’ve learned in recent years, body temperature also has a significant effect on circadian rhythms. Studies in the past several years have shown that changes to core body temperature transmit strong signals to the circadian clocks that tick away in cells throughout our body.
The most pronounced bio-time temperature shift happens in the later part of the day and evening, as the body prepares itself for sleep.
Beginning in the late afternoon and extending into night, a number of temperature-related changes occur within the body. The body begins to produce less heat internally. It also begins to push heat to its extremities—the hands and feet, the top of the head. Blood vessels on the skin become larger (a process called vasodilation) in order to release heat. These physiological changes work to lower your core body temperature. In turn, with that reduction 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.
Hot temperatures make it harder for the body to shed heat and cool itself. And that poses a number of problems for sleep. 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. Even for good sleepers, it’s harder to maintain a strong sleep efficiency when it’s hot out. Research shows different insomnia symptoms—both difficulty falling asleep, and difficulty staying sleep, become more common in hot weather.
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 deprive your body of these highly restorative, beneficial sleep stages.
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.
We face other costs from the sleeplessness that a heatwave brings. Excessive heat and humidity raise the risks for heat-related illnesses, including sunburn, heat rash, heat exhaustion, and heat stroke. New research shows that sleep deprivation can lead to changes in our perception of the symptoms of heat-related illnesses. When we’re tired, we’re likely to experience the discomfort, disorientation, stress and pain of heat sickness more acutely.
The combination of sleep deprivation and high ambient temperatures also can affect our cognitive performance, particularly our focus and attention, as a recent study of shift workers demonstrates. Sleeplessness is well understood to interfere with a whole range of our cognitive function, from learning, to memory, to decision making and the processing of emotions. Combine lack of sleep with days of hot, humid weather—when even nights don’t cool down enough to get some real relief—and it’s little surprise we don’t feel or perform at our best.
So, what’s to be done? It is possible to sleep well throughout a heatwave. It just takes a little planning and some attention to sleep-friendly habits.
- Get ahead of the heat spike
You know that keeping your sleep environment cool is key to getting a good night’s rest at any time of year. In the summer—and especially when the temperature soars—it takes more effort to keep your home and bedroom comfortable. Don’t wait until lights out to Keep the shades closed during the day, run fans and air conditioning well before your bedtime, and you’ll keep your bedroom a cool retreat. The ideal room temperature for sleeping for most people is around 65-67 degrees Fahrenheit. But everyone is different, so pay attention to identifying exactly what temperature best suits your sleep.
- Stay hydrated
Dehydration interferes with our ability to sleep well. People who snore or have obstructive sleep apnea are especially prone to the sleep-depriving effects of dehydration. Did you know that being sleep deprived can also make you more at risk for dehydration? That’s because a hormone—vasopressin—that’s an important regulator of fluid in the body is produced in abundance during some of our later sleep cycles. Cutting sleep short can cause a lack of vasopressin that makes dehydration more likely—especially when it’s hot and you’re sweating a lot. Drink water consistently throughout the day. If you drink too much of your daily water in the evening, you’ll wind up waking up to go to the bathroom throughout the night.
- Eat a light dinner
For a lot of us, the heat takes away our appetite. And the middle of a heatwave is the last time any of us want to turn on our ovens. The light dinners that suit hot nights can make it easier for your body to transition into sleep. On the other hand, when your body is busy digesting a large meal, falling asleep can take longer—and you’re more likely to sleep restlessly.
- Nap smart
If hot weather cuts into your sleep time, you’re going to feel it during the day. Naps aren’t always an option, but if your schedule allows, a daytime nap can help you replenish that lost rest. There’s a way to nap—and also a when to napping—that make the difference between a nap that refreshes you and a nap that disrupts your evening sleep. I’ve written in depth about napping before. The quick takeaway? There are two optimal nap durations: 20 minutes (a quick snooze that energizes you for a few hours without post-nap grogginess) and 90 minutes (the length of a full sleep cycle, with all its restorative benefits.) The ideal time for a nap? Approximately 7 hours after you wake for the day.
- Take a warm shower or bath
A warm soak before bed helps a lot of us fall asleep. That’s because it enhances the evening temperature drop the body undergoes in the evening, as the body prepares to sleep. It might sound counterintuitive to recommend a warm shower or bath on our hottest nights, but it’s likely to soothe you, and help transition your body toward sleep. It doesn’t need to be a scorching hot shower—go ahead and select a gently warm temperature that feels good on your skin.
- Use a cool—not a cold—compress (or one of my favorite sleep tools, the Chilipad)
Cooling your skin on a hot day provides some instantaneous relief from the heat. A cool compress on your shoulders, neck, arms, and torso can help you relax before bed and make your way more easily into sleep for the night. Why do I recommend cool, not cold? The body releases heat through its extremities at night, as part of the natural cooling that accompanies nightly rest. To do so effectively, the blood vessels on the skin become larger, what’s known as vasodilation. Cold temperatures on the skin have the opposite effect—they narrow blood vessels, making it harder for the body’s internal heat to be released. A tepid, or a slightly warm compress can relief discomfort without compromising your body’s ability to shed heat before and during sleep.
I often recommend to my patients they use Chilipad, a sleep system that enables you to control your body heat throughout the night. Chilipad regulates the temperature of your mattress surface, helping you sleep soundly and wake up feeling alert. One great feature of Chilipad? It lets bed partners adjust their body temperatures separately. And Chilipad isn’t just for summer—it can help you maintain an ideal body temperature for sleep all year long.
- Double down on your healthy sleep habits when it’s not scorching hot
If you run up a sleep debt throughout the summer, you’re less prepared to weather the tricky sleeping that comes with a heatwave. Take advantage of all the pleasant summer days and nights to strengthen your sleep routine, and pay down any sleep debt. Stick to consistent bed times and wake times. Avoid over-consumption of caffeine, and steer clear of caffeine and other stimulants after about 2 p.m. Eat healthfully, and get plenty of exercise.
Here’s a big one: manage your light exposure carefully. One of the sometimes-overlooked contributors to sleep problems during the summer is all the additional daylight. The long summer days are great, but the early sunrise and late sunset mean the body gets less of the darkness it needs to make melatonin. In addition to interfering with sleep, short nights and prolonged daylight can also lead to “the summer blues,” a reverse seasonal affective disorder that happens in the summer.
By all means, enjoy the sunshine. And get plenty of it in the morning, when it can help make you alert and strengthen the circadian bio rhythms that also help you sleep. It’s always important to manage your evening light exposure, and it’s especially so in the summer. I just wrote about this, and how blue light blocking glasses can help you limit unhealthful, sleep depriving light exposure without compromising any of the ways you like to relax at night (hello, Netflix).
Stay cool, and sleep well!
Michael J. Breus, PhD, DABSM
The Sleep Doctor™