Sleep has a profound impact on human health, improving attention, memory, emotional regulation and work performance, and reducing the risk of disease. But over the past few decades, the amount of sleep that people get has declined substantially. A 2017 study of some 690,000 children from 20 countries found that nightly sleep duration fell by more than an hour from 1905 to 2008. Nearly one-third of adults in the United States sleep for less than six hours per night, and sleep duration has dropped since the 1960s for adults in Japan, Russia, Finland, Germany, Belgium and Austria.
“Society as a whole has changed quite a lot,” says Theun Pieter van Tienoven, a researcher at the Free University of Brussels who studies gender division of labour, daily routines and sleep sociology. “We tend to want to do much more than we can handle in our daily life.” According to van Tienoven, digitalization and globalization have driven many people to cram more activities into their daily schedules, often at the expense of sleep. The use of social media at night and fear of missing out have been shown to reduce sleep quality, perhaps because people stay up later to engage in social activities, and exposure to blue light from screens has a damaging effect on their sleep–wake cycles, or circadian rhythms.
Getting a good night’s sleep can be difficult, especially for busy scientists, who often work long hours, nights and weekends while juggling their personal lives and responsibilities outside the workplace. “I am one of those people with multiple roles and thus at high risk of having bad sleep habits or lower quality of sleep due to the mental load of all those roles,” says van Tienoven, who is also a field-hockey player and coach and a parent to three children, aged seven, eight and ten. His wife is a paediatrician. Nature asked sleep researchers and physicians to share their insights into how sleep affects the brain, and how busy scientists can improve their sleep habits.
“Historically there wasn’t as much interest in sleep, because for many centuries people thought sleep was akin to death,” says Mark Wu, a physician and neurologist at Johns Hopkins University in Baltimore, Maryland, who studies sleep disorders and the genetic mechanisms that regulate sleep. “What really spurred the development of sleep research was a discovery that there are very specific brainwave patterns that occur during sleep,” he says. The discovery was made in 1929 by the German psychiatrist Hans Berger, using a technique called electroencephalography (EEG), which measures electrical activity in the brain. Eight years later, a group of scientists used the same technique to identify different stages of sleep.
Today, most researchers describe sleep as occurring in two main phases: rapid-eye-movement (REM) sleep and non-REM sleep. Non-REM sleep comprises three stages, known as N1, N2 and N3, and these are characterized by specific brainwave patterns that reflect neural activity. These three stages of non-REM sleep plus REM sleep comprise the sleep cycle, and people who sleep well typically experience four to six cycles every night.
When someone is awake, an EEG of their brain shows a mixture of high frequencies in which a lot of neurons are firing, but not in a completely coordinated way, Wu says. Then, as the person starts to go into a light sleep, or the N1 stage, those frequencies slow down into a pattern called theta waves. This stage typically lasts for one to ten minutes.
During the second phase of sleep, N2, body temperatures drop, heart rates and breathing slow down and muscles relax. Brainwave frequencies now oscillate between slower waves and short bursts of neural activity. This phase generally lasts 10–25 minutes during the first sleep cycle, but gets progressively longer in later cycles, ultimately comprising about half of a person’s sleep time each night.
Slow-wave sleep occurs in the N3 stage, which is characterized by slow, high-amplitude waves called delta waves. This 20–40-minute phase makes up about one-quarter of a person’s sleep time. “When we think about what makes us feel good, this is the form of sleep that really does that,” says Wu. Some evidence suggests that slow-wave sleep plays a crucial part in restoring energy levels, repairing and growing tissues, clearing waste and boosting the immune system. Slow-wave sleep also aids learning and memory consolidation.
Neural activity picks up when the body enters REM sleep, which is when dreaming occurs. In this phase, which comprises one-quarter of sleep time each night, the brainwaves follow a beta-wave pattern. “You have mixed frequencies, so it looks like you’re awake,” Wu says. As the night progresses, the periods of slow-wave sleep shorten and REM sleep increases. REM sleep is important for cognitive functions, such as emotional regulation, memory and creative problem-solving.
Although researchers have a good understanding of the various phases of sleep, it’s been challenging to pinpoint what sleep does, says Wu. “Sleep is the only major behaviour and physiological process for which we don’t understand the function. It’s a very challenging problem because sleep is a brain-wide network effect.”
“Sleep is multidimensional,” says Lisa Matricciani, who studies how sleep affects human health and well-being at the University of South Australia in Adelaide. One dimension is the quality of sleep. This is often measured through an individual’s perception, but it can also be quantified by the number and duration of night-time awakenings, as well as by how long it takes for the person to fall asleep at night.
Matricciani tries to get at least eight hours of sleep each night. She achieves this by maintaining a consistent sleep schedule and avoiding coffee after 3 p.m., which helps her to fall asleep more easily. “Often people think alcohol promotes sleep. It may make you fall asleep faster, but it causes more disrupted sleep,” she adds.
The average adult needs 7–8 hours of sleep per night. “But it’s a bell curve. Some only need three; others need 12 or 14 hours,” says Wu. Along with differences in sleep duration, individuals vary in when they feel sleepy and when they feel awake. For instance, ‘early birds’ prefer to go to bed early and rise early, whereas ‘night owls’ like to stay up late and sleep late.
The quality of a person’s sleep is driven by a variety of factors, including their circadian clock, homeostatic drive (a process that increases a person’s desire to sleep as the day progresses, and decreases during sleep) and stress. When a person’s sleep schedule isn’t synchronized with their circadian clock, the quality of their sleep can decrease, causing them to feel tired and groggy. People who work night shifts, or who try to sleep during the day, often struggle with their sleep schedules being out of tune with their circadian clocks, as do those with jet lag.
Arousal, whether from stimulants such as coffee and cigarettes, excitement or stress, also affects the quality of sleep because it increases the time needed to fall asleep, reduces the duration of sleep or causes sleep to be more fragmented. Wu avoids caffeine and bright lights at night to help improve the quality of his sleep (see ‘Science-backed tips for good sleep’). All exposure to light can suppress the secretion of melatonin, a hormone involved in regulating the body’s sleep–wake cycle, but exposure to blue light from electronic devices and energy-efficient lamps has the largest impact. Wu often tells his patients to avoid taking naps, especially right before bedtime, because it can prevent them from feeling tired. Getting exercise is a great way to enhance sleep, but the timing matters, because the process of exercising releases hormones such as adrenaline and cortisol that can keep people awake. Wu recommends that his patients do moderately strenuous exercise for 30–45 minutes a day, but not too close to bedtime — a recommendation that he also follows.
Stress from family conflicts, work deadlines and other challenges can increase the body’s levels of cortisol, a hormone that regulates stress, metabolism, wakefulness and other bodily functions, and can disrupt sleep patterns. “I always make jokes with my students, saying you should never fight with your husband or wife at night — do that in the morning,” says Monica Andersen, who researches the effects of sleep on sexual function and skin conditions at the Federal University of São Paulo, Brazil.
Poor-quality sleep can affect cognitive function in ways that translate to the workplace and home life. People who are sleep-deprived lose attention more readily, for instance.
“Poor sleep results in poor judgement, errors in decision-making and decreases in reaction time and motor performance. Many workplace accidents are due to poor sleep,” says Hrudananda Mallick, a physiologist at Shree Guru Gobind Singh Tricentenary University in Gurugram, India, who almost always goes to bed at 10.30 or 11 p.m. and wakes up at 6 a.m..
People who are sleep-deprived can also become less aware of how their performance is being impaired. “Chronic sleep deprivation has been likened to excessive alcohol consumption. In a similar way that a person who is intoxicated may not realize their performance is affected, a person who is sleep-deprived may become less aware that their physical and cognitive abilities are compromised,” Matricciani says.
As well as affecting cognition, low-quality sleep is associated with obesity, heart disease, diabetes and early death. “Sleep habits can influence lifestyle, like dietary habits and physical activity, smoking and drinking,” says Zhilei Shan, a nutritional epidemiologist at Huazhong University of Science and Technology in Wuhan, China. People who are sleep-deprived are likely to feel tired and be less inclined to exercise, whereas those who sleep for too long could miss opportunities to exercise and eat breakfast, he says. Although Shan typically sleeps for nine hours each night on weekdays and ten hours each night at weekends, he prioritizes exercising during the day and reading at night to relax and stay healthy.
Creating a comfortable sleep environment is really important for ensuring a good night’s sleep, Matricciani says. According to the US National Sleep Foundation, a charity based in Washington DC that advises on sleep-related matters, dark, quiet and cool environments (16–19 °C) can support good sleep, and the use of earplugs, sleeping masks and room-darkening shades can block noise and light. The foundation also recommends eating a light dinner at least two hours before bedtime, to allow time to fully digest it.
Along with these activities, implementing a sleep schedule can go a long way toward enhancing sleep, says van Tienoven. This means going to sleep and waking up at the same time every day, even on weekends, and establishing a schedule for daily events and tasks, such as eating breakfast and going to work. Reducing stress by engaging in calming activities, such as reading, meditation and journalling, can also improve the quality of sleep. “There are some suggestions, although not everyone agrees, that people write down everything that worries them before bed, like ‘I have to pay my rent, I have to organize my agenda, I have to clean the house.’ The moment that you put your thoughts down, you forget about them until the next morning,” says Andersen.
Andersen says that she tries her best to maintain healthy sleep habits, but that it’s been more challenging since she became a mother. “Before my daughter was born, I would typically go to bed around 10 p.m. to wake up at 5.30 or 6 a.m.. Now, I wake up at least three times per night to breastfeed,” she says.
Van Tienoven makes daily to-do lists to ensure that he finishes all his important tasks before going to sleep; he doesn’t respond to work e-mails after 9 p.m.; and he discusses plans for the next day with his wife. They often prepare by making lunches for their children and packing bags for school, work and sports before they go to bed. “I believe in strong bedtime rituals — for kids but also for grown-ups — and I try to go to bed with an ‘empty’ mind,” he says.