Masashi Yanagisawa on the Mysteries of Sleep: Why Living Things Sleep, and What Happens When They Don't

Yanagisawa says that, from the standpoint of sleep science, "why do people sleep?" is a hard problem that could even be called an "eternal mystery." The fact that not just humans but every animal studied so far sleeps is a mystery in itself.

More precisely, the creatures that sleep are "those with a nervous system." Research advanced from around 2000, tracing back through the vertebrates and showing that fish sleep too. It was further reported that invertebrates with completely different body plans, such as insects, nematodes, squid and octopuses, also sleep. Insects, squid and octopuses have small but genuine brains and engage in complex behavior. According to Yanagisawa, however, their brains are not homologous to ours. Brains were clearly invented more than once over the course of evolution. If both kinds sleep anyway, the question arises of where sleep originated.

As Yanagisawa recalls, a paper published around 2017 showed that jellyfish sleep too. Jellyfish have nerve cells throughout their bodies; they swim by moving their bells and catch food by stinging with their tentacles. But they have nothing that could be called a central nervous system. Even so, jellyfish show behavior that can be called sleep. From this, Yanagisawa says, it can be argued that sleep was invented before the brain. Still, it is not well understood why every creature with a nervous system needs to sleep, even at the cost of danger.

In mammals, sleep divides broadly into REM sleep and non-REM sleep. Yanagisawa describes REM sleep as, in a sense, a "mismatched sleep." During REM sleep we have vivid dreams, such as dreams with a storyline or dreams accompanied by emotion. That does not mean the sleep is light. In the sense that it takes a strong stimulus to wake someone, it is deep sleep. During REM sleep, the muscles of the whole body also go completely limp, a state known as "REM atonia," and motor output drops to nearly zero. The only exceptions are the muscles involved in breathing and the eye muscles. The name REM (Rapid Eye Movement) comes from the movements of the eyes beneath the eyelids, which are thought to follow the scenes of the dream. Meanwhile, the output of the autonomic nervous system becomes active, and breathing and heart rate become irregular. This combination is why it is called "mismatched."

Non-REM sleep, as the name suggests, is sleep that is not REM, the state of sleeping peacefully, Yanagisawa explains. We have hazy dreams but rarely clear ones. Brain activity does not drop; rather, the brain is performing "maintenance work in an offline state," and from the outside it looks as though the person is sleeping quietly. Breathing and heart rate become very regular. Non-REM sleep has light, intermediate and very deep stages.

The program's narration adds that memory organization is thought to take place in both kinds of sleep. Non-REM sleep is said to move recent memories into long-term memory, while REM sleep is said to be involved in consolidating skills that are hard to put into words, such as sports or playing a musical instrument. However, the detailed mechanisms of which kind of sleep processes which memories, and how, are not yet understood.

Yanagisawa says it is certain that sleep is essential for retaining what we learn over the long term. If you study well before bed and then get a proper night's sleep, that memory becomes consolidated. From a sleep science perspective, cramming overnight is not recommended at all. Sleeping well at night, rather than sleeping briefly, is extremely important for consolidating memories over the long term, and he stressed that "it's important for students preparing for exams to sleep well."

As for dreams, Yanagisawa explains that during sleep a kind of "replay" takes place of the day's events, things we have learned and scenes that left an impression, and part of that replay becomes conscious as a dream. Why this happens is not yet understood. What is known, he says, is that replay during sleep is necessary for organizing and consolidating memories. In addition, many psychologists propose that the vivid, emotional dreams of REM sleep play a role in building stress tolerance. According to this theory, "rehearsing" stressful situations that could happen in real life within unpleasant dreams may build resistance to stress and make us less emotionally shaken.

Because we cannot ask animals what they dream about, we cannot say for certain that animals dream. According to the narration, however, responses shared with humans, such as eye movements during sleep, suggest that many animals are highly likely to dream as well.

Yanagisawa's example was the octopus. It was recently reported that octopuses also have two types of sleep, "quiet sleep" and "active sleep." Whether it is appropriate to call these REM and non-REM is not known, he says. During active sleep, the body stays still and asleep, yet the skin has been observed to change color in sudden bursts. One interpretation is that something like a dream is happening and the octopus is camouflaging itself to match the scene. Yanagisawa added the caveat, however, that since we cannot ask an octopus what it is dreaming, strictly speaking we do not know.

Yanagisawa also cited the example of a certain species of penguin that takes extremely short sleeps of about 4 to 5 seconds each, thousands of times a day, adding up to 8 hours, 10 hours or even more of sleep in total. Dolphins and whales can put the left and right hemispheres of the cerebrum to sleep separately, so from the outside they appear to stay awake and keep swimming almost 24 hours a day. Animals have acquired diverse ways of sleeping suited to their environments. Even so, why they must sleep at all remains a mystery.

Yanagisawa sums it up by saying sleep holds two great mysteries. One is the fundamental function of sleep: why every creature with a nervous system has to sleep. There is still no clear answer to this.

The other, related mystery is "what sleepiness actually is." People cannot stay awake indefinitely. The longer we stay awake, the more pressure toward sleep builds up, and we subjectively feel sleepier. The only way to relieve it is to sleep, and during sleep the pressure gradually dissipates. This may sound obvious, but Yanagisawa says almost nothing is yet known about what concrete state in the brain this "sleepiness" corresponds to.

On the other hand, we are beginning to understand what happens when we lose sleep. According to Yanagisawa, even a single night of sleep deprivation causes brain performance to drop sharply. Brain performance after pulling an all-nighter is comparable to that after drinking 3 to 4 portions of alcohol, with one portion defined as 20 g of pure alcohol. That is equivalent to a bottle of wine, nearly 2 liters of draft beer, or 3 to 4 go of sake, close to being completely drunk.

Even without pulling an all-nighter, getting 6 hours of sleep a night for about two weeks drags performance down to the same level as an all-nighter. With 4 hours of sleep a night, it takes 5 to 6 days to reach the all-nighter level, and after two weeks performance falls to about the same as going three nights without sleep. What Yanagisawa finds "frightening" is that subjective sleepiness at that point rises only to about the level of a single all-nighter at most. Subjective sleepiness does not keep pace with chronic sleep deprivation. You could say people get used to it.

Performance here includes not only the speed of work and information processing and the avoidance of oversights and mistakes, but also emotional control. People brood, get irritated or become quick to anger. What Yanagisawa takes particularly seriously is the impairment of the ability to step back and look at things from a broad perspective, of metacognition, meaning the ability to objectively evaluate one's own behavior, and of creativity.

The effects on the body are also significant. Yanagisawa listed mental health problems that can lead to depression in the worst cases; metabolic syndrome, including obesity, high blood pressure, diabetes and hyperlipidemia; and an increased risk of cardiovascular diseases such as stroke and heart attack that follow from it. The immune system also weakens, making people more susceptible to infections. There are even reports that antibody levels rise less after vaccination. Over the long term, the risk of certain cancers also rises. There are "clear results," he says, showing that the risk of future dementia increases as well. Moreover, since it is known that these diseases also worsen sleep, it is easy to fall into a vicious cycle.

Accumulated sleep deprivation is also called "sleep debt." According to Yanagisawa, a certain amount of debt can be repaid. Many people follow a pattern of being short on sleep on weekdays and paying some of it back by sleeping longer on weekends. But two days are probably not enough; it is said that fully repaying accumulated debt takes at least 3 to 4 days.

Yanagisawa explains that so-called "sleeping in to stock up" is not saving sleep but paying back debt that has piled up. When sleep needs are completely met, a person cannot sleep any more. So strictly speaking, sleep cannot be saved up.

The program frames knowing how much sleep you need as something to figure out before thinking about sleep quality. According to Yanagisawa, the amount of sleep that is necessary and sufficient varies between individuals by about plus or minus one hour in standard deviation. The differences by age are also large: the average for people around 20 is close to 8.5 hours, whereas at 70 it falls below 6.5 hours, a difference of about two hours.

Individual differences within the same age group are known to be highly heritable. Yanagisawa said the heritability of sleep duration is higher than the heritability of height. Very rarely, there are people whose required sleep time is extremely short. These are short sleepers. Yanagisawa stressed that this is an innate constitution, and that no one can train themselves to become a short sleeper.

As a way to find out how much sleep you need, Yanagisawa recommends sleeping as much as you can for four nights in a row. Since many Japanese people routinely carry sleep debt, they sleep very long on the first night. On the second night, having repaid part of the debt, they sleep less, and on the third night less still. Around the fourth night, it settles and stops changing. The sleep duration on the third and fourth nights onward is the amount of sleep that is necessary and sufficient for that person. If you can secure these conditions during a long weekend or summer vacation, ideally for four nights and at least for three, he encouraged people to give it a try.

Yanagisawa says he often tells people that "the most important thing about sleep quality is quantity." First secure enough sleep time, and then improve the quality. Japanese people, especially younger generations, are across the board short on sleep, but the quality of their sleep while they are asleep is actually good, and they get plenty of deep sleep. According to Yanagisawa, this is a state in which the brain is crying out for help: a feedback mechanism kicks in to sleep as deeply as possible within the short sleep, and it is known that the deepest non-REM sleep increases. So if you want to improve quality, the only way is to secure quantity.

Yanagisawa calls the so-called "golden time for sleep" "basically a lie." It used to be said that 10 p.m. to 2 a.m. was the golden time, and recently it has often been said that "the first three hours of sleep are what matter." It is true that the deepest non-REM sleep (slow-wave sleep, N3) appears during roughly the first three hours after falling asleep. Maintenance such as organizing memories and transferring them to long-term memory takes place then, and growth hormone is secreted. Growth hormone not only makes children's bodies grow but is also important for adults in maintaining the cells of bones, muscles and skin. Yanagisawa acknowledges that all of this is fact. But over the past 10 to 20 years or so, sleep science has found that the REM sleep that increases in the second half of the night, and the light non-REM sleep of the second half, are all important too. The conclusion, he says, is that there is no golden time, and the whole night's sleep matters.

Another piece of advice Yanagisawa rejects is "even when you can't sleep, lying down with your eyes closed gives you a good deal of rest." Unless you actually sleep, sleep debt is never relieved. What's more, this advice can in some cases be a gateway to insomnia. Once or twice is fine, but if you repeatedly lie in bed tossing and turning without sleeping night after night, the body learns through what psychology calls conditioning that "my bed is a place where I can't sleep." Because this becomes a gateway to full-blown insomnia, Yanagisawa considers the advice mistaken.

As for improving quality once quantity is secured, Yanagisawa first describes the bedroom environment as "dark, quiet, and at a comfortable temperature until morning." As a rule, keep it pitch dark and quiet while you sleep, and do not turn off the air conditioning, summer or winter.

Regulating the body clock is also very important, he says. The ideal is regular sleep: going to bed and getting up at almost the same times every night, including days off. When you get up in the morning, open the curtains, let bright light into your eyes and eat breakfast. Conversely, from around 8 or 9 p.m. into the night, the ideal is to avoid bright light and spend time in an environment dim enough to feel a little dark. Yanagisawa pointed out that nighttime lighting in Japanese homes is too bright by global standards.

For the time right before sleep, he says a bedtime ritual (bedtime routine) is "really important." If you follow a similar pattern of behavior every night, the body learns that it means "it's time to sleep," and you actually become sleepy. What works differs from person to person, and there is no magic prescription that works for everyone. You just need to do a sequence of things that relaxes you.

It is often said that smartphone light is harmful, but Yanagisawa says there is little basis for this. Today's smartphones and tablets automatically dim after dusk or in dark places, and their colors shift toward yellow to reduce blue light. So "don't look at your phone in bed because blue light is bad" has no scientific basis, he says.

What Yanagisawa sees as the problem is interactive content such as games, social media chats and "likes," and short videos. Short videos keep demanding input, and AI serves up one engaging piece of content after another, so you get excited wondering what will come next. If you keep at it, you never get sleepy, falling asleep is put off, and before you know it an hour, or worse, two hours have passed. He says this is a fairly major cause of lost sleep. Yanagisawa admitted that they themselves have slipped up, starting to watch short videos and finding that 30 minutes had passed without noticing.

As for a nightcap, he says it is "actually the worst" thing for sleep. Alcohol has a direct sedative effect, so you can fall asleep, and that is what fools people. But sleep with alcohol in your system has less deep sleep, becomes unstable, involves more awakenings during the night, and is of extremely poor quality. Yanagisawa says people who cannot sleep without a drink every night should see a doctor. There are now new types of prescription sleep medications that are effective, safe and not habit-forming, and sleeping with those is far better for both the brain and the body than sleeping with alcohol, he says.

One of those new sleep medications is the orexin receptor antagonist. Orexin is a substance that maintains wakefulness in the brain, and it was Yanagisawa who discovered it.

According to Yanagisawa, the discovery dates back to around 1995–96. The brain has many "keyhole molecules" (receptors) responsible for transmitting information between nerve cells, and many had been found whose corresponding "key molecules" were unknown (orphan receptors). Thinking that finding an unknown key would make it possible to start a new area of biology, they searched, and what they found was orexin.

At that point, however, all that was known was that a new substance existed in the brain. The nerve cells that produce orexin were found only in the hypothalamus, deep in the brain, in a region known to be involved in appetite. When orexin was injected into a rat's brain, the rat immediately began eating a lot, and when rats were fasted to make them hungry, orexin production in the brain increased. So Yanagisawa and colleagues thought it might be a brain chemical that controls appetite, and reported as much in their first paper in 1998. Appetite control is also an important issue medically, and it was very exciting, he says.

Next, Yanagisawa and colleagues created mice that could not produce orexin. If it were a substance that stimulates appetite, mice lacking it should eat less and become thin. But their food intake, body weight and mortality were almost no different from those of normal mice. Yanagisawa recalls being at a loss: "We finally found a new brain chemical, and it makes no difference whether it's there or not?"

So they filmed the nighttime behavior of the nocturnal mice with an infrared camera. Mice that had been moving about actively would suddenly freeze, sometimes toppling over onto their sides, and then get up and start moving again after 30 seconds to a minute. Such "behavioral arrest attacks" were seen frequently. At first, Yanagisawa and colleagues thought it might be a type of epilepsy without convulsions.

When they measured brain waves, there were no abnormalities characteristic of epilepsy; instead, they found that the moment the mice fell over, they abruptly shifted from wakefulness to sleep. As the narration explains, REM sleep normally comes after passing through non-REM sleep, but in these mice that order broke down, and they collapsed due to the atonia that accompanies REM sleep. Yanagisawa says they realized this was exactly the same symptom as narcolepsy, a form of central hypersomnia in humans.

There were abnormalities in non-REM sleep as well. According to Yanagisawa, when mice fall asleep normally, they prepare first: if materials are available they build a nest, curl up inside and groom themselves before sleeping. They do not suddenly flop over and fall asleep. But these mice would suddenly fall over and enter non-REM sleep. Both REM attacks and non-REM attacks occurred. In other words, they could not properly maintain wakefulness, which is precisely human narcolepsy. In this way, without aiming for it, they discovered that a lack of orexin causes narcolepsy. Put the other way around, it became clear that orexin is essential for keeping the brain properly awake. Later research went on to show that orexin is something like the "boss" of the brain's arousal systems. Yanagisawa says they had the sense that this was truly a blue ocean.

Around the same time, Emmanuel Mignot's group at Stanford University had arrived at the same place entirely independently, from the opposite direction. They had maintained a line of dogs with hereditary narcolepsy for many years, and in searching for the causative gene, they identified it as one of the orexin receptors. Yanagisawa and colleagues started from the orexin gene, and knocking it out in mice caused narcolepsy. Mignot and colleagues started from the disease narcolepsy and found that the causative gene was the orexin receptor. It was simultaneously discovered in two species that abnormalities in a ligand and its receptor lead to the same disease, and Yanagisawa recalls being very excited.

From the idea that suppressing orexin's action should induce sleep, a new type of sleep medication was developed. According to Yanagisawa, the first one became clinically available in Japan about 10 years ago. It is safe and not habit-forming, a "sleep medication you can graduate from," he says.

Yanagisawa adds that what is now attracting attention in the news is orexin agonists. Drugs that bind to orexin receptors and substitute for orexin are finally becoming available, he says. In fact, right after it became clear that a lack of orexin causes narcolepsy, Yanagisawa approached pharmaceutical companies, since it was theoretically obvious that an agonist would be a breakthrough treatment for narcolepsy. But at that time, about 25 years ago, no pharmaceutical company took it seriously. Narcolepsy was thought to be a relatively rare disease. Yanagisawa explains that it occurs in about 1 in 1,000 people, so it is by no means rare, but it is also not as common a disease as high blood pressure or diabetes.

Yanagisawa received the Breakthrough Prize in Life Sciences in 2023. Known for having the largest prize money in the world, about $3 million per prize, it is a glamorous award whose ceremony is held on a red carpet attended by Hollywood celebrities, they say. They also received the Clarivate Citation Laureate award, given to researchers whose papers are widely cited and have far-reaching impact. It is a fact that quite a number of Breakthrough Prize winners have gone on to win the Nobel Prize, and Yanagisawa says it was an honor to receive what is seen as a stepping-stone award. More than that, Yanagisawa emphasized that although sleep is a field of great social importance, it had never before received a truly major scientific award. Yanagisawa has always shared these awards with Mignot, and says it is wonderful in itself that sleep science received a major prize.

On their career, Yanagisawa says they moved to the United States at 31 (in 1991). It was not a study-abroad stint; they were recruited to the University of Texas Southwestern Medical Center as a PI (principal investigator), the head of a lab, and had an independent lab of their own for the first time. According to the narration, after discovering orexin in the United States, Yanagisawa had given little thought to returning to Japan. The turning point came in 2010, when they were selected for a major national basic research support program and opened a lab at the University of Tsukuba.

They then applied to a national program to build world-class research centers, proposing the concept of "creating an American-style department inside Japan." The proposal was well received and selected, and they launched WPI-IIIS, dedicated to sleep research. It now has more than 20 independent labs and is run with nearly 350 people, including graduate students. Its defining feature is that every lab conducts basic sleep research in some form; Yanagisawa said it is by far the largest institute in the world dedicated to basic sleep research, and that they take pride in it being one of the world's leading institutes in terms of research level as well.

Yanagisawa says that "artificial hibernation is no longer science fiction." According to the narration, in 2020 a research team led by Sakurai at WPI-IIIS focused on a very small number of nerve cells in the brain called "Q neurons." As Yanagisawa explains, when this very small group of nerve cells in the anterior hypothalamus is specifically stimulated in mice, which do not normally hibernate, the mice suddenly enter a hibernation-like state. Their body temperature drops to nearly room temperature, they stop moving and become cold, and at first glance they appear dead. But when they recover, they return to normal in perfect health. Sakurai and colleagues have succeeded in mice and in larger rats, and are now trying to confirm whether the same thing happens in primates such as marmosets.

The narration notes that because humans are known to have the same type of nerve cells as Q neurons, humans may still retain the capacity to hibernate. According to Yanagisawa, looking at the mammalian family tree, hibernating species are scattered here and there across various branches, and even among primates there is a hibernating monkey somewhere like Madagascar. From this, one can take the view that the latent capacity for hibernation lies at the root of mammals, and whether it is used differs from species to species, he says.

If this were achieved in humans, in the distant future there could be science-fiction-like applications such as space travel or sleeping for decades to travel into the future. But what Yanagisawa looks forward to most is a change in emergency and critical care medicine. If a person could be put into a hibernation-like state on the spot in cases of serious illness or accident, the progression of the condition could be slowed, effectively stopping time until the patient is transported to a place where appropriate treatment is available. They are watching this closely because it could bring about a tremendous change in emergency medicine.

The program takes up insomnia and sleep apnea syndrome as representative sleep disorders. According to Yanagisawa, insomnia is a very common disease. Four to five people in the population, and one in three among the elderly, complain of insomnia to some degree, and if daytime impairments such as sleepiness or lack of energy persist chronically, insomnia is diagnosed. (Correction for fidelity: one in four to five people in the population.)

Yanagisawa says insomnia is probably a disease that is basically unique to humans. Animals other than humans probably never suffer from the worry of being unable to sleep. Why people become unable to sleep is hard to pin down because insomnia is fundamentally a subjective disease. What patients feel, that they cannot sleep, often diverges from whether they are objectively not sleeping. It is very common for brain wave measurements to show that someone slept soundly while the person themselves felt they had not slept.

Yanagisawa says that globally, the most effective treatment for insomnia is not sleep medication but cognitive behavioral therapy. A common pattern among insomnia patients is a vicious cycle: because they can't sleep, they think they need to go to bed early and get into bed around 10 p.m., can't fall asleep, grow increasingly anxious, and become even less able to sleep. Cognitive behavioral therapy does the opposite, having patients go to bed late and get up early to shorten their time in bed. Many patients are taken aback: "I'm telling you I can't sleep, and you're telling me not to sleep?" They complain of being unbearably sleepy during the day, but eventually they become able to at least sleep soundly while in bed, and that becomes the starting point for their anxiety about sleep to fade and for them to become able to sleep. Yanagisawa explained that this is not all there is to it, but that is the basic idea. Treatment usually takes many months and is demanding, but if it goes well, the effects last for years, and it is said that it can basically lead to a complete cure.

Yanagisawa says that sleep apnea syndrome is even more troublesome than insomnia. Insomnia patients are troubled enough to see a doctor, but many people with apnea have no subjective symptoms. According to the figures Yanagisawa cited, 22 million people in Japan have sleep apnea, and of those, 9.4 million potentially have moderate or more severe cases requiring treatment alone. It is an "insane" number: about 10% of the population has apnea requiring treatment, and it is said to affect about one in three men from middle age onward.

Stopping breathing 30 or more times per hour, meaning more than once every two minutes, is classified as severe. According to Yanagisawa, there is an epidemiological study showing that if severe apnea is left untreated, about 40% of people die over the following ten-plus years from cardiovascular events such as stroke, heart attack, aortic dissection, or sudden death from arrhythmia during sleep. Even among those who do not die, many are left with aftereffects from stroke. Yet the current situation is that more than 90% are not receiving treatment, he says.

As a reason people do not get treated, Yanagisawa points to the perception that "it's just snoring." Without a bed partner, no one even points out the snoring. Most people who snore loudly should assume they have apnea to some degree. Nocturia, getting up to use the bathroom two or three or more times during the night, is also a frequently occurring subjective symptom, and people with it should first suspect apnea, he says. Until old age, it is overwhelmingly more common in men. Also, the image of it as a disease of overweight middle-aged men is not accurate; especially among Asians, it is common in people who are thin but have a small upper or lower jaw. The tongue has nowhere to go, so when they sleep on their backs it falls backward and blocks the airway. Yanagisawa described it as a disease that may be even more common than insomnia, yet patients do not go to the doctor.

Yanagisawa has set midnight to 7 a.m. as their "core sleep time," and tries as much as possible not to use that time for anything other than sleep, including on weekends. There are days when they can't keep to it, they say.

They say people should "really stop" hitting the alarm's snooze five or ten times in a row. The sleep between repeated snoozes is very light and a waste of time. They recommend setting one alarm about 10 minutes before you actually need to get up, limiting snooze to once, for a total of two alarms. There are reports that a single snooze is effective, and papers have been published showing that if you shift into light sleep once and then sleep until the last moment, you can wake up more smoothly at the next alarm, they say.

Yanagisawa, too, occasionally has nights when stress, worries or a pile of things to do keep them from sleeping. On those occasions they say they half give up, but what they make a point of doing is deep breathing. In a method they call "15-second breathing," they breathe in slowly and deeply through the nose over about 7 seconds and breathe out over about 8 seconds. One round trip takes 15 seconds, and four repetitions make exactly one minute. They recommend doing it while counting their own heartbeat, saying that they drift off to sleep before they know it.

According to Yanagisawa, various surveys have repeatedly shown that the Japanese are the most sleep-deprived people in the world. The narration states that in international comparisons, average sleep time in Japan is about one hour shorter than in Europe, and that even within Asia, where sleep times are generally short, Japan's are strikingly short. Yanagisawa put the reason simply: Japanese people make light of sleep and do not give it priority in their lives.

What Yanagisawa says is particularly serious is sleep deprivation among children. Probably less than 10% of junior high and high school students in Japan get a medically sufficient amount of sleep. There is "a mountain of" data showing that sleep deprivation also ruins studying. As an example, Yanagisawa cited a paper from Tohoku University that took MRI scans of the brains of about 300 children of various ages and measured the volume of the hippocampus, which is important for memory. The conclusion was simple: children who tended to be short on sleep had smaller hippocampi, and children who slept well had larger ones. Yanagisawa expressed concern that sleep deprivation in childhood may be a lifelong handicap.

Finally, asked what sleep is for humanity, Yanagisawa answered that it is "a gift from God." Among the many mammals, only humans have the ability to sleep deeply and continuously in one block at night, they say. According to some theories, humans became able to sleep soundly once they grew smart enough to secure a safe environment at night, and as a result became even smarter; in other words, the ability to sleep deeply and continuously co-evolved with the advanced brain. Since this way of sleeping is unique to humans, cherish it, and do not take nighttime sleep lightly: that is Yanagisawa's message. The question of why humans sleep still has no answer.