How to Sleep on Hot Summer Nights: Science vs. Myth

Heatwaves are like kryptonite to sleep, but our ancient ancestors dealt with wild temperature variations, and so can you

This article is part of a Wise & Well Special Report: Extreme Heat and Human Health.

Cavemen must have had all kinds of problems drifting off at night. Sleeping on dirt floors, rocks for pillows, sabertooth tigers prowling around the cave entrance. But at least they scored a cool, dark place — two among several key ingredients to successful sleep.

Not all our ancestors slept in caves, of course. Throughout prehistory, humans slept outside, cowboy style, yet without jeans or jammies or even miracle-cooling 900-thread percale sheets. How’d they endure hot summer nights?

Fact is, summer slumber has always been a rocky endeavor, and humans have never slept as much as we’ve been led to assume.

As I explain in my book, Make Sleep Your Superpower, we’ve been fed a big lie — by pharmaceutical companies and organizations they’ve helped fund — that we need eight hours or more of sleep every night. Failure to achieve that magical threshold supposedly leaves us all walking around like sleep-deprived zombies. The myth is further fed by claims that our ancestors slept a whole lot more than what any reliable evidence indicates.

The result: Eight hours is an ingrained benchmark in much scientific literature and popular writing.

The reality is that while sleep needs vary by individual, most of us modern humans don’t need eight-plus hours, several experts have told me. What many of us could truly benefit from is better sleep. And while summer can indeed be sleep’s kryptonite, heat is just one of several factors that can keep you up at night. By understanding what really goes into good or bad sleep, most people can improve the quality of their sleep, and also the duration — regardless of the season — with side effects like improved mood, increased productivity and better overall health and happiness.

Once upon a time…

Jerome Siegel, PhD, chief of neurobiology research at the University of California, Los Angeles, examined the sleep patterns of three modern hunter-gatherer societies in different parts of the world: the Hadza in Tanzania; the San in Namibia; and the Tsimane in Bolivia. These people live much like our ancient ancestors, without lights or alarm clocks or other modern conveniences.

They sleep between 5.7 and 7.1 hours nightly, on average, year-round. But they sleep less in summer, Siegel found. The Hadza, for example, get about one hour less sleep in summer compared to winter.

Unlike many Westerners, these modern hunter-gatherers suffer very little insomnia, Siegel found. They seem to get the sleep they need.

The message is not that we should try to sleep less, or that 5.7 hours is healthy. Ample research finds extreme risks to physical health, mental well-being and longevity among people who sleep less than six hours nightly. Rather, it’s become clear that eight hours is not a scientifically supported threshold that most people need to aim for. The majority of us — and there are exceptions owing to physical illnesses, sleep disorders and other issues — can function just fine, and be perfectly healthy, on less than eight and even as little as seven or so.

“The more we look, the less we can profess eight hours to be normal,” writes Harvard University evolutionary biologist and paleoanthropologist Daniel Lieberman, PhD, in his book Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding.

The elements of good sleep, summer or otherwise

The cool, dark place that some cave dwellers may have sometimes experienced really would be conducive to better slumber… coupled with other important factors. A nice pillow would help, I imagine. The lack of smartphones and social media, evening work emails and 24/7 news, was certainly good for sleep.

But preparing for a good night’s sleep—back then and still today—actually starts first thing in the morning.

Exposure to daylight — as much as you can get, and especially early in the day — triggers suppression of the get-sleepy hormone melatonin. That fosters alertness and normal daytime functioning and starts the clock ticking in preparation of the evening production of melatonin. It keeps the overall daily sleep/wake cycle on track.

That body clock, well tuned to the natural world, helped our ancestors nod off naturally, typically a few hours after sunset, then wake up at dawn, Siegel found.

Natural daylight, which is many times brighter than what you experience in your home or office, is said by many sleep experts to be the chief governor of our circadian rhythm, the chemical and electrical timing machine that keeps the body clock properly set. But temperature plays a vital role in setting the body clock, too. The 24-hour cycle of warm days and less-warm nights has left us evolutionarily programmed to react to temperature cues (along with light cues) to promote awakeness and sleepiness.

It all revolves around biology. In order to sleep well, the body temperature must drop. When everything is in synch, that happens naturally each night, on schedule, as the heart rate and breathing rate slow and we drift into dreamland.

Siegel’s research highlights the importance of both light and temperature for sleep, said Andrea Spaeth, PhD, an experimental psychologist and director of the Sleep Lab at Rutgers University.

“Hunter-gatherer societies experience significant fluctuation in temperature across the day and across seasons because they spend all of their time outside,” said Spaeth, who wasn’t involved in Siegel’s research. “Modern humans are not experiencing the same flux. I think the more we can get outside during the day to experience natural light and temperature, and get fresh air, the better it is for our sleep and health overall.”

Siegel takes the logic a step further.

“I think that the loss of daily cycles of temperature in our air-conditioned, heated environment may be a cause of high insomnia rates,” he told me.

We’ve become so conditioned, we literally can’t take the heat, and yet, confoundingly, AC isn’t a silver bullet for sleep problems.

Serious summer sleep stats

Our ancient body clocks remain intact, even if we screw with them daily by avoiding natural stimuli and immersing ourselves in drab daytime office lighting and then artificial evening illumination from screens and room lighting. Still, in the United States, people on average sleep about 25 minutes less in summer than winter. In Japan, the difference is around 11 minutes, and similar seasonal divergences are found across Europe.

Night-to-night temperature differences have a strong effect, too.

When the overnight low temperature outside exceeds 86 degrees Fahrenheit (30 Celsius), people get on average 14 minutes less sleep per night than on cooler nights, based on data on 47,000 adults in 68 countries on all continents except Antarctica. There are notable differences within the findings:

• Women lose more sleep in the heat than men.
• Older people also are more negatively affected by hot nights.
• The effect is three times as strong among people in poorer countries than in rich nations, perhaps due to lack of AC, the researchers suggest.
• Incremental temperature increases — a degree or two — cause greater sleep loss for people in warm climates compared with those in cooler climates.

“We provide the first planetary-scale evidence that warmer-than-average temperatures erode human sleep,” Kelton Minor, PhD, a data science researcher at Columbia University who led the research while at the University of Copenhagen, said in a statement. “We show that this erosion occurs primarily by delaying when people fall asleep and by advancing when they wake up during hot weather.”

While the conclusions seem clear and logical, and the “erosion of sleep” is technically accurate, keep in mind that the Hadza experience a notably bigger difference in summer sleep time — roughly 60 minutes less compared to winter. So sure, heat can reduce sleep duration, but that’s only natural.

And here’s the real eye-opener: Duration is only one aspect of sleep effectiveness.

If you get high-quality sleep, any missed minutes won’t be as devastating as they would be if you’re sleeping lightly, tossing and turning all night.

Here’s why: The best nights of sleep include multiple cycles through four stages. In the deepest stage, brain waves slow way down. It’s then that the body’s muscles, organs and other pieces and parts get repaired and rejuvenated at the cellular level. Also only in the deep phase, the physical brain is literally cleaned up by a garbage collection and disposal mechanism called the glymphatic system.

How to beat the heat

Several types of sleep kryptonite can inhibit quality. But if you’re asleep — even if fitfully — how do you know if you’re getting sufficient, truly restorative deep sleep? The best gauge is how you feel the next day. Signs your sleep sucks:

• Afternoon tiredness
• Lack of focus during the day
• Excessive anxiety or stress
• Irritability or a short temper

You may go through the motions, but you lack the vigor, enthusiasm and clear-headedness cultivated by sufficient, quality, deep sleep.

The path toward better sleep—any time of year—can start with any one of a number of steps. Master one, sleep a little better, and you’ll have the energy and focus to tackle another. Since the heat is on these days, let’s start with the science of sleeping in cooler room temperatures, which can definitely offer a strong assist but for which there’s no clear prescription.

Ideal room temperature for sleep has been put by various researchers at between 60 and 70 degrees Fahrenheit (15.6 to 21.1 C). That’s a big range, and it may seem quite frigid to some people, or too expensive to maintain during a heat wave. Moreover, setting the thermostat can be really challenging for couples, since each of us can have significantly different preferences based on wildly varying internal thermostats and perceptions of warm and cold.

All that in mind, here’s some practical advice, if you think heat is your problem and you have the means to cool your bedroom down without starting a fight with your bed partner:

“If someone is having an issue with their sleep, starting out with an ambient temperature of about 66 degrees [18.9 C] is a good start, and then they can make adjustments up or down from there to figure out what’s best for them,” Spaeth said in an email.

If you still feel overheated or sweat a lot, consider your bedding. “Adjusting the fabric and weight of bedding depending on the time of year can be helpful for regulating temperature during sleep,” Spaeth said.

You may already have the right pajamas and bedding for the job. Try crisp-feeling sheets made of cotton, linen or bamboo, advises W. Chris Winter, MD, a neurologist at Charlottesville Neurology and Sleep Medicine. Avoid polyester. Same for PJs.

You might even try a hot bath an hour or two before bedtime, research has shown. Seriously? In this heat? Yep. A hot bath triggers the body to cool itself down. It’s akin to pointing a hair dryer at a thermostat to get the AC going.

Maybe it’s not just the heat

But if you can’t avoid hot nights and if hot baths, AC or an armada of portable fans isn’t helping you sleep, or if your bed partner’s ideal temp is your worst nightmare, it’s time to look at some other forms of sleep kryptonite (and there are many, including serious sleep disorders that are best dealt with by doctors or sleep clinicians) that may underlie your troubles.

• Alcohol is atrocious for sleep, especially deep sleep (more on this below).
• Caffeine late in the day can keep you up unnaturally late at night.
• Lack of physical fitness is notoriously bad for sleep.
• Unmitigated stress is a lousy bedfellow.

Improving on any single behavior or condition can jumpstart your path to better sleep. Among the most fruitful places to begin, according to mountains of research: eat better, exercise more, get outside, cut back on booze if you drink, and learn how to deal with stress effectively.

When you put a little intention into any or all of these fixes, and make sleep a priority by chipping away at the kryptonite, then melatonin is more able to do its job: make you sleepy in the evening. When the stars align, a process called sleep pressure also kicks in — based simply on how many hours you’ve been awake — and builds to the point that, along with the effects of melatonin, you can’t keep your eyes open.

Your sleep quality will improve, and duration may to increase, too, as you spend less time staring at the ceiling even if you spend the same amount of time in bed. Or maybe you won’t need more sleep, and you’ll learn to rise earlier. When I sleep particularly well — according to the app on my watch and also based on how I feel — I tend to sleep less.

Combine your new behaviors with a consistent bedtime and wake time, seven days a week, and all the good habits build upon one another. As I explain in my book, an extremely positive cycle develops, a new superpower:

You’ll fall asleep faster, sleep more deeply, and wake up more refreshed and ready to face the day. That’ll make you more productive, able to hunt down that mastodon or get in the modern equivalent of a workout, and less apt to be stressed out by negative events. By bedtime, you’ll be more than ready to crawl into your cool, dark place — or a warm one if that’s all you have — and sleep like a caveman, maybe even like a baby.