Sleep has a profound impact on physical health. It can improve attention, memory, emotional regulation, and work performance, as well as reduce the risk of illness [1, 2]. However, over the past few decades, people's sleep time has significantly decreased. A study in 2017 of 690,000 children in 20 countries [3] found that nighttime sleep duration decreased by more than an hour from 1905 to 2008. Nearly one - third of adults in the United States sleep less than 6 hours per night [2]. Since the 1960s, the sleep duration of adults in Japan, Russia, Finland, Germany, Belgium, and Austria has been declining [4].

"The whole society has changed a lot," said Theun Pieter van Tienoven, a researcher at the Free University of Brussels who studies the gender division of labor, daily routines, and the sociology of sleep. "We always want to accomplish more tasks in our daily lives than we are capable of." Van Tienoven believes that digitalization and globalization have filled many people's days with various activities, often at the expense of sleep. Browsing social media at night and the fear of missing out have been proven to reduce sleep quality. Perhaps it's because people stay up late to participate in social activities, and the blue light from electronic screens has an adverse effect on the sleep - wake cycle (i.e., the circadian rhythm) [5].

Getting a good night's sleep may not be as simple as it sounds, especially for busy scientists. These scientists usually work long hours, stay up late, and work overtime on weekends while also taking care of their personal lives and fulfilling off - work responsibilities. "I have multiple roles, so it's easy to develop poor sleep habits or have my sleep quality decline due to these mental stresses," said van Tienoven, who is also a hockey player, coach, and has three children aged 7, 8, and 10. His wife is a pediatrician. Nature invited sleep researchers and doctors to share their advice on how sleep affects the brain and how busy researchers can develop good sleep habits.

Not "Almost Dead"

"Historically, people had little interest in sleep because for centuries, they thought sleep was almost the same as being dead," said Mark Wu, a doctor and neurologist at Johns Hopkins University in the United States who studies sleep disorders and the genetic mechanisms of sleep regulation. "The impetus for the development of sleep research mainly came from the discovery that there are special brainwave patterns during sleep." This was discovered by German psychiatrist Hans Berger in 1929 using electroencephalography (EEG) technology, which can measure the electrical activity of the brain [6]. Eight years later, a team used this technology to discover the different stages of sleep [7].

Today, most researchers divide sleep into rapid eye movement (REM) sleep and non - REM sleep. Non - REM sleep can be further divided into three stages, N1, N2, and N3, based on specific brainwave patterns that reflect neural activity. These three non - REM sleep stages, plus REM sleep, form a complete sleep cycle, and people who sleep well generally experience 4 - 6 such cycles per night [8].

Wu said that when a person is awake, the brain's EEG shows various frequencies of neuronal firing, which are not fully coordinated. Then, as the person begins to enter light sleep, the N1 stage, these frequencies slow down into a theta - wave pattern [8]. This stage usually lasts 1 - 10 minutes.

In the second stage of sleep, N2, body temperature begins to drop, heart rate and breathing slow down, and muscles relax. The brainwave frequency now oscillates between slower waves and brief neural activities. This stage usually lasts 10 - 25 minutes in the first sleep cycle and gradually lengthens in subsequent cycles, accounting for about half of a person's nightly sleep duration.

Slow - wave sleep occurs in the N3 stage, which is characterized by slow, high - amplitude delta waves. This 20 - 40 - minute stage accounts for about one - quarter of sleep time [8]. "When we say that sleep makes people feel good, we're talking about this stage," Wu said. Some evidence shows that slow - wave sleep plays a key role in restoring energy, tissue repair and growth, removing waste, and enhancing the immune system [8]. Slow - wave sleep also promotes learning and memory consolidation.

When the body enters REM sleep, neural activity also increases, and dreaming begins. In this stage, which accounts for one - quarter of nightly sleep time, the brainwaves are in a beta - wave pattern. "There are all sorts of frequencies, and it looks as if you're awake," Wu said. As the night deepens, the duration of slow - wave sleep shortens, and REM sleep increases [8]. REM sleep is important for cognitive functions such as emotional regulation [9], memory [9], and creative problem - solving [10].

Although researchers have a good understanding of the different stages of sleep, it is still difficult to clarify the specific effects of sleep. "Sleep is the only important behavior and physiological process whose function we don't understand. It's a very challenging problem because sleep produces effects on the whole - brain network," Wu said.

A Good Night's Sleep

"Sleep has multiple dimensions," said Lisa Matricciani, who studies the impact of sleep on physical health and well - being at the University of South Australia. One of these dimensions is sleep quality. This is generally measured by personal perception, but it can also be quantified by the number and duration of nighttime awakenings and how long it takes to fall asleep.

Matricciani tries to ensure at least 8 hours of sleep per night. To achieve this, she goes to bed on time and avoids coffee after 3 p.m., which helps her fall asleep faster. "People always think that drinking alcohol can promote sleep. Alcohol may help you fall asleep faster, but it makes your sleep easily interrupted," she said.

On average, adults need 7 - 8 hours of sleep per night [2]. "But this is a normal distribution. Some people may only need 3 hours of sleep, while others need 12 to 14 hours," Wu said. In addition to differences in sleep duration, individuals also have different feelings about when they feel sleepy and when they wake up. For example, morning people like to go to bed early and wake up early, while night owls like to go to bed late and wake up late.

There are many factors that affect sleep quality, including an individual's circadian rhythm clock, homeostatic drive (the process where sleepiness gradually increases during the day and decreases during sleep), and stress. If a person's sleep schedule is out of sync with their circadian rhythm clock, their sleep quality will decline, and they will be more likely to feel tired and fatigued. People who need to work night shifts or sleep during the day, like those who need to adjust to jet lag, always have to overcome the problem of inconsistent sleep schedules and circadian rhythm clocks.

Arousal - whether from coffee, cigarettes, or stimuli such as excitement or stress - also affects sleep quality because it increases the time it takes to fall asleep, reduces sleep duration, or causes fragmented sleep [11]. Wu refuses caffeine and dims the lights at night to improve his sleep quality (see "The Scientific Secrets of Sleeping Well"). Any light exposure inhibits the secretion of melatonin, a hormone involved in regulating the body's sleep - wake cycle, and the blue light from electronic products and energy - saving lamps has the strongest inhibitory effect on melatonin [12]. Wu always advises his patients not to take naps, especially close to bedtime, because it reduces their feeling of fatigue. Physical exercise is a good way to promote sleep [13], but the timing of exercise is crucial because exercise releases hormones such as adrenaline and cortisol, which can keep people awake. Wu advises his patients to do 30 - 45 minutes of moderate exercise every day, but not close to bedtime, and he follows this advice himself.

The Scientific Secrets of Sleeping Well

Although you may want a cup of coffee to ward off afternoon sleepiness, caffeine can make it difficult for you to fall asleep and affect the length and intensity of slow - wave sleep. This may be because caffeine blocks the binding of adenosine to brain receptors, and adenosine regulates the body's sleep - wake cycle.

Yoga nidra is an ancient way to relax the body, mind, and emotions. It helps to extend sleep time and improve the sleep efficiency and quality of people with chronic insomnia.

Listening to music when falling asleep can also improve sleep quality, especially for people with mild sleep problems [21]. Gaelen Thomas Dickson and Emery Schubert from the University of New South Wales in Australia believe that this may be because music can pull people out of the inertia of deep thinking and high alertness. But they wrote that audiobooks do not have this effect. "It seems that some mysterious effects of music still need scientific explanation."

The stress of family problems, work tasks, and other challenges can increase the body's cortisol levels. Cortisol is a hormone that regulates the body's functions such as stress, metabolism, and wakefulness, and it can also interfere with sleep patterns [14]. Monica Andersen, who studies the impact of sleep on sexual function and skin condition at the Federal University of São Paulo, said, "I always joke with my students that you should never argue with your husband or wife at night. If you have to argue, do it in the morning."

Loss of Efficiency

Poor sleep quality can affect cognitive function, thereby affecting an individual's work and family life. For example, people with insufficient sleep are more likely to have difficulty concentrating [15].

"Poor sleep can lead to misjudgments, wrong decisions, slower reactions, and decreased sports performance. Many work accidents are caused by insufficient sleep," said Hrudananda Mallick, a physiologist at Shree Guru Gobind Singh Tricentenary University in India. He goes to bed at 10:30 or 11 p.m. on time and gets up at 6 a.m.

People with insufficient sleep are also less likely to realize the impact of this on their performance [16]. "Chronic sleep deprivation has been found to be related to excessive drinking. Just as drunk people are unaware that their behavior is affected, people with insufficient sleep are also less likely to notice the decline in their physical and cognitive abilities," Matricciani said.

In addition to affecting cognition, poor - quality sleep is also associated with obesity, heart disease, diabetes, and premature death [2]. Shan Zhilei, a nutritional epidemiologist at Huazhong University of Science and Technology in Wuhan, said, "Sleep habits can affect lifestyle, such as eating habits, physical exercise, smoking, and drinking." He said that people with insufficient sleep are more likely to feel tired and reluctant to exercise, while those who sleep too long may miss the time for exercise and breakfast. Although Shan Zhilei sleeps 9 hours per night on weekdays and 10 hours on weekends, he arranges all his exercise during the day and reading in the evening to ensure physical and mental relaxation and good health.

Optimal Sleep

Matricciani said that creating a comfortable sleep environment is very important for ensuring high - quality sleep. According to the recommendations of the National Sleep Foundation in the United States, a dark, quiet, and cool environment (16°C–19°C) is beneficial for sleep. Using earplugs, eye masks, and indoor blackout curtains can block out noise and light. The foundation also recommends having a light dinner and not eating at least 2 hours before going to bed to give the body enough time to digest.

In addition to these activities, making a sleep plan can also help improve sleep, van Tienoven said. This requires going to bed and getting up on time every day, even on weekends, and also arranging daytime activities and tasks, such as having breakfast and going to work. Releasing stress through calming activities such as reading, meditation, and writing a diary can also improve sleep quality [17, 18]. "Here are some suggestions. Although not everyone agrees, writing down all the things will make people more anxious before going to sleep, such as 'I have to pay the rent, I have to write a plan, I have to clean the house.' Only by putting these things aside first can you temporarily forget them before tomorrow morning," Andersen said.

Andersen said that she has always adhered to healthy sleep habits, but it has been difficult to maintain since she had a child. "Before my daughter was born, I always went to bed at 10 p.m. and got up at 5:30 or 6 a.m. Now, I have to get up at least three times a night to breastfeed," she said.

Van Tienvoen writes down what he needs to do each day to ensure that he can complete all important work before going to bed at night. He stops replying to work emails after 9 p.m. He also discusses the next day's plan with his wife. They prepare their children's lunches for the next day and pack school, work, and sports bags before going to bed. "I believe in the benefits of going to bed on time, for both children and adults. I go to sleep with a 'cleared' mind," he said.

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