Sleep is a peculiar habit. It forces us, and countless other organisms on the planet, to forgo food, mating, and avoiding predators for substantial chunks of every day. And while we can all attest that when we don’t get enough sleep, we feel it, it’s not totally clear why. Indeed, it might surprise you to learn that exactly why we need sleep in the first place remains a mystery.
Such unknowns mean that researchers send back frequent bulletins from the frontiers of science: Sleep deprivation causes dangerous molecules to accumulate in the gut, work with mice and fruit flies reveals, perhaps explaining why a total lack of sleep can kill lab animals. Sleep lets waste products drain from the brain, one set of studies shows — but newer work challenges that. Sleep improves our ability to consolidate new memories, though perhaps not more than simple, quiet wakefulness does.
What’s not controversial is that many of us just aren’t sleeping enough: According to a CDC survey, over one-third of Americans report not getting the recommended amount of shut-eye. This should not be taken lightly, as those getting insufficient sleep “are at a much greater risk for diabetes, stroke, hypertension, obesity, depression, anxiety, cancers,” says neuroscientist Steven Lockley, a visiting professor at the University of Surrey in the United Kingdom.
We spoke with experts to find out what’s important for all of us to know about sleep and how, perhaps, to improve our experience of it. Here are five takeaways.
1. Tired? You’re not alone.
Are we living in an age of unprecedented sleepiness? Though modern phone use may have exacerbated problems with getting to sleep, it’s not as clear-cut as you might think.
A major review of studies between 1960 and 2013 found that people slept about the same amount regardless of the year. It doesn’t seem to be about tech either: A 2015 study had people in three pre-industrial societies in Namibia, Bolivia, and Tanzania wear activity trackers to record sleep habits, and found that they did not sleep more than their industrialized counterparts — the duration ranged from 5.7 to 7.1 hours a night. And anxiety about lack of sleep has long been a perennial problem. In 1894, a British Medical Journal editorial included this tidbit: “The subject of sleeplessness is once more under public discussion. The hurry and excitement of modern life is quite correctly held to be responsible for much of the insomnia of which we hear.”
Regardless, the fact remains that many today would like to be sleeping more. For adults, pressures of commutes, child care, and multiple jobs may mean they are not able to follow their body’s cues as closely as they’d like. Sleep deprivation is a problem for teenagers as well. The recommendation for children ages 13 to 18 is eight to 10 hours a night. But two-thirds of middle and high schoolers responding to a CDC survey slept less than that, which the health agency says could affect mental health, attention, and behavior, among other outcomes.
There are racial and ethnic disparities to consider as well. “This is a bigger problem for underrepresented populations,” says Tom Scammell, a Beth Israel Deaconess neurologist and a professor of neurology at Harvard Medical School. The most sleep-deprived in the CDC survey were Black and Pacific Islander adults, with roughly 1 in 2 respondents saying that they needed more sleep.
The consequences of sleeplessness today could be more serious than they were in the past, says Lockley, who is also the cofounder of Timeshifter, an app aimed at helping travelers reduce the symptoms of jet lag. “You wouldn’t have had to worry about drowsy driving 150 years ago,” he says.
2. You’re probably a poor judge of your own sleepiness.
The American Academy of Sleep Medicine recommends adults get a minimum of seven hours of sleep a night. That means you have to be in bed longer than seven hours, Lockley says, because normal night sleep is peppered with tiny awakenings.
But if you’re regularly getting less sleep than that and feel fine, should you be worried? Scammell says people are not very good at assessing whether they are suffering from sleep deprivation. In his experience, asking more questions often reveals red flags, like drinking four cups of coffee a day or needing an afternoon nap.
“When people get into these chronic situations, what they consider normal may not be representative of how their day should be,” he says. Lockley points to studies suggesting that sleep-deprived subjects perform worse on cognitive tests without realizing it. “Your ability to assess is impaired,” he says. “You often think you are doing better than you are.”
People with severe sleep apnea, a condition which causes them to stop, then restart, breathing at night, may feel foggy during the day. But they may also think they sleep wonderfully, says Andrew Wellman, director of the Sleep Disordered Breathing Lab at Brigham and Women’s Hospital. He tells a story of a patient who insists he is a good sleeper, while his assessment results tell a different story, full of disturbed sleep. “The reason he thinks he sleeps fine,” Wellman says, “is he is so tired that he falls asleep instantly.”
Wellman recommends this test: Do your eyelids feel heavy during the day? If they do, that’s a better indication of sleep deprivation than whether you feel “fine.”
3. Can’t sleep? The best treatment isn’t a pill.
The gold standard treatment for insomnia — having trouble falling asleep or staying asleep — is not sleep medication, which can cause side effects such as cognitive impairment and dizziness. Cognitive behavioral therapy for insomnia (CBT-I), which may include patients recording how long they spend in bed asleep and awake, dimming the lights before bed, and working with a therapist to build habits that help them feel tired at the right time, is as effective or more effective than sleeping pills and has fewer side effects, a 2019 study showed.
It can be challenging to find a CBT-I practitioner, Scammell says. But even without a therapist, there are apps that aim to help people gain some of these skills. Sleepio, for instance, has been found to be useful in treating anxiety and insomnia in two large randomized controlled trials.
For insomniacs, aiming for a certain number of hours a night can be counterproductive, Wellman says. When anxiety about getting enough sleep makes it harder to sleep, it’s time to take off your activity tracker and start focusing on good habits, rather than numbers. “It’s not one size fits all,” he says, of sleep advice. It all depends on context.
When it comes to children, improving habits — such as dimming lights in the house and avoiding screens for several hours before bedtime — should be the first step in improving sleep, experts say.
A recent trend has seen parents giving the hormone melatonin to their children, in hopes of helping them sleep. According to one study, 6 percent of preschoolers and nearly 20 percent of older children had been given melatonin, many of them every night. This may seem harmless, as melatonin — a hormone produced by the body in the hours before sleep — is available without a prescription. But there is almost no data about the safety of long-term melatonin use in children, Scammell warns. What’s more, Lockley says, in many animals, melatonin is known to impact the reproductive system, raising questions about whether it should be considered safe for children without further study.
4. Light is a powerful drug. Let it help you.
The timing of sleep is controlled in part by your internal sleep drive, which keeps track of how long it’s been since you slept. It’s also controlled by the body’s circadian clock, which keeps track of the daily cycle of light and dark. Both systems are affected by light, a stimulant that can keep your sleep drive from getting you to drift off, Lockley says. Light at the wrong time can shift your clock so you are no longer in tune with your environment. (Think trying to sleep in a very different time zone.)
Lockley says he and his colleagues have exposed volunteers briefly to bright indoor lights, and found something striking. Even 15 seconds of very bright light was shown to shift the internal clock, he says. The clocks of those who’d seen the bright light, in other words, had shifted more than those who hadn’t.
Blue light, emitted from fluorescent and LED lights and back-lit electronic screens, has more impact on circadian rhythms than other types of light, according to the National Institute for Occupational Safety and Health. Indeed, during the evening, exposure to even moderate amounts of blue light can cause your body to delay the production of melatonin. Those who have trouble falling asleep should consider avoiding screens on televisions, smartphones, and other digital devices before bed, NIOSH recommends.
Red light, however, has little effect on circadian rhythms, which is why doctors may suggest reading by a warm-colored light before bed rather than using your phone.
5. Sleep is older than brains.
In all the anxiety over getting enough sleep, it may help to reflect on the bigger picture. It’s easy to assume that sleep evolved for the brain — after all, we feel “clear-headed” after a good night’s rest — and for many years, sleep scientists believed this was the case.
But something more unexpected may be closer to the truth. The road to our current understanding began in the 1970s, when biologist Irene Tobler began documenting behavior in cockroaches — whose nervous systems have two brain-like clusters of neurons — that looked a lot like sleep.
The insects had periods of stillness during which they were hard to rouse, and if they were roused, they had to make up for it later with more rest. In the decades since, studies based on Tobler’s criteria have revealed sleep in organisms as diverse as fruit flies and roundworms. Those same sleep behaviors can be seen in Cassiopea jellyfish, which have no centralized brain, just a nervous system. An even simpler creature, the hydra, was spotted sleeping in 2020.
That suggests two things: One, that sleep is perhaps more than a billion years old, when the last shared ancestor of humans and hydras lived. Two, sleep may not have evolved as a maintenance procedure for the thinking appliance in our skulls. For us, sleep keeps us alert, supports cardiovascular health, and likely allows our bodies to take care of things they can’t while we are awake. For the earth’s first sleeper, we can only imagine what it meant.
Think of that the next time you get into bed — what you are about to do is ancient and mysterious, and we are only just beginning to understand its profound role in our lives.
Veronique Greenwood is a science writer based in the United Kingdom. Send comments to [email protected].
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