Slow-wave sleep also aids learning and memory consolidation. 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 8. “When we think about what makes us feel good, this is the form of sleep that really does that,” says Wu. This 20–40-minute phase makes up about one-quarter of a person’s sleep time 8. Slow-wave sleep occurs in the N3 stage, which is characterized by slow, high-amplitude waves called delta waves. 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 8. Brainwave frequencies now oscillate between slower waves and short bursts of neural activity. This stage typically lasts for one to ten minutes.ĭuring the second phase of sleep, N2, body temperatures drop, heart rates and breathing slow down and muscles relax. 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 8. 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. 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 8. Non-REM sleep comprises three stages, known as N1, N2 and N3, and these are characterized by specific brainwave patterns that reflect neural activity. Today, most researchers describe sleep as occurring in two main phases: rapid-eye-movement (REM) sleep and non-REM sleep. Eight years later, a group of scientists used the same technique to identify different stages of sleep 7. 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 6. “What really spurred the development of sleep research was a discovery that there are very specific brainwave patterns that occur during sleep,” he says. “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. 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. “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. 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. 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 5. “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. “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. Nearly one-third of adults in the United States sleep for less than six hours per night 2, and sleep duration has dropped since the 1960s for adults in Japan, Russia, Finland, Germany, Belgium and Austria 4. A 2017 study 3 of some 690,000 children from 20 countries found that nightly sleep duration fell by more than an hour from 1905 to 2008. But over the past few decades, the amount of sleep that people get has declined substantially. Sleep has a profound impact on human health, improving attention, memory, emotional regulation and work performance, and reducing the risk of disease 1, 2. Koalas can sleep for 20 hours a day, partly because they have a low-energy diet.
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