Sleep is a spontaneous, reversible state of rest that occurs periodically in higher vertebrates, characterized by reduced responsiveness to external stimuli and temporary interruption of consciousness. Throughout a person’s life, roughly one-third of the time is spent sleeping. When people are asleep, their brains and bodies can rest, recover, and rejuvenate. Adequate sleep is essential for daily work and study. Scientifically improving sleep quality is crucial for ensuring people’s ability to function normally at work, study, and in everyday life.

　　Basic Information

　　Chinese name: Sleep, to sleep Chinese: Sleep, to sleep Name: Sleep, to sleep

　　Foreign name: sleep

　　Reason: Organize memories

　　Category: Biological Behavior

　　Sleep causes

　　Sleep is a state in which sensory and motor activities temporarily cease due to internal bodily needs, yet can be immediately awakened upon receiving appropriate stimulation. After gaining an understanding of brain electrical activity, scientists came to believe that sleep results from reduced physiological activity in animals caused by the functional activity of the brain, and that appropriate stimulation can restore the body to a fully awake state.

　　Sleep is an active process—a rest essential for restoring one’s energy. There is a specialized central nervous system that regulates sleep and wakefulness. During sleep, the human brain simply switches to a different mode of operation, allowing it to conserve energy and facilitating both mental and physical recovery. Adequate sleep is the best form of rest; it not only serves as the foundation for maintaining health and physical strength but also guarantees high levels of productivity. The heightened excitability of nerve cells, which continuously receive and respond to internal and external stimuli, prevents interference among unprocessed stimulus associations by means of selective inhibition—this is precisely what alleviates fatigue. Poor sleep quality, on the other hand, arises when the degree of sensory gating is insufficient or when sleep duration is inadequate to fully process and integrate these stimulus associations. Excessive and prolonged drowsiness, meanwhile, represents a pathological state of overly intense and prolonged sensory gating. All these conditions reflect inadequate neural control. During sleep, as spontaneous activity diminishes, the body’s physiological state also undergoes recovery.

　　The primary function of sleep is manifested in the brain: during sleep, particularly through dreaming, the brain stitches together and organizes scattered memory fragments into a coherent whole.

　　Physiological changes

　　Sleep is often an unconscious, pleasurable state that typically occurs when we lie in bed and allow ourselves to rest at night. Compared with the waking state, during sleep, a person’s contact with the surrounding environment ceases, conscious awareness fades away, and one loses the ability to consciously control what one says or does. While asleep, muscles relax, neural reflexes become weaker, body temperature drops, heart rate slows down, blood pressure slightly decreases, metabolic rate slows down, and gastrointestinal motility markedly weakens. At first glance, a sleeping person appears still and passive—but this is only an illusion. If we were to perform an electroencephalogram (EEG) on a sleeping person, we’d find that the electrical impulses emitted by brain cells during sleep are no less active than those during wakefulness. This clearly demonstrates that the brain is not resting at all. Just like a beehive at night—though from the outside it may seem as though all the bees have returned to their hives and are resting, in reality, every single bee is busily working through the night to produce honey.

　　After a normal adult falls asleep, the first stage entered is the slow-wave sleep phase, typically progressing through stages 1, 2, 3, 4, 3, and 2 in sequence, with each stage lasting anywhere from 70 to 120 minutes. This is followed by the rapid eye movement (REM) sleep phase, which lasts about 5 to 15 minutes. Thus ends the first sleep cycle transition, after which the slow-wave sleep phase begins again, leading to the next REM sleep phase. This cycle repeats continuously throughout the night. On average, there are 4 to 6 such cycles during an entire night’s sleep. During these cycles, the duration of the slow-wave sleep phases gradually shortens, with stage 2 becoming the predominant one, while the duration of the REM sleep phases gradually lengthens. If we take the total duration of sleep as 100%, slow-wave sleep accounts for approximately 80%, whereas REM sleep accounts for about 20%. By arranging the different sleep stages and wakefulness states in chronological order according to their occurrence, we can construct a sleep graph, which provides a visual representation of the dynamic changes occurring across the various sleep stages.

　　Sleeping style

　　Sleep consists of two alternating phases: one is the slow-wave phase, also known as non-rapid eye movement (NREM) sleep, and the other is the rapid-eye movement (REM) sleep, during which rapid eye movements occur and dreaming is common. REM sleep primarily serves to restore physical energy, while NREM sleep mainly helps to recover mental energy.

　　Slow-wave sleep

　　Based on the characteristics of human brainwave activity, this stage is typically divided into four distinct phases, corresponding to the progression of sleep from light to deep. In phase 1, low-voltage brainwaves with a mixed range of frequencies—predominantly in the 4–7 cycles per second range—are observed. This phase often occurs at the onset of sleep and during brief awakenings throughout the night.

　　Non-rapid eye movement (NREM) sleep is more prevalent in the first half of the night. This is because, after a day of intense mental activity, most people experience significant cognitive fatigue, placing considerable stress and damage on brain cells and tissues. Naturally, our biological clock prompts us to sleep, allowing the brain’s temperature to drop and ushering in a repair phase—this process is crucial for sustaining life. In the latter half of the night, rapid eye movement (REM) sleep becomes more prominent, which is closely linked to learning and work. As we encounter numerous experiences—both positive and negative—throughout the day, these must be encoded into memory. NREM sleep plays a key role in this process by facilitating the acquisition of new information and integrating daily emotional experiences.

　　Paradoxical sleep

　　Paradoxical sleep, also known as REM sleep, is characterized by the regular, rhythmic side-to-side movements of the two eyeballs beneath the eyelids during sleep. These movements occur in a predictable, alternating pattern throughout the night and are referred to in sleep medicine as sleep cycles. On average, there are 4 to 5 sleep cycles per night. Non-REM sleep predominates during the first half of the night, while it virtually disappears in the latter half; in contrast, REM sleep gradually increases throughout the second half of the night.

　　It is an aroused state that occurs cyclically during sleep. On the electroencephalogram, it manifests as fast-frequency, low-amplitude brain waves similar to those observed during wakefulness. The autonomic nervous system shows heightened activity, with increases in heart rate, respiration, and blood pressure; cerebral blood flow and oxygen consumption both rise. In males, this phase is accompanied by penile erection. Moreover, sleepers frequently turn over in bed, and muscles in the face and at the ends of the fingers and toes exhibit occasional twitching. In experimental animals, recordings have also revealed that the firing activity of individual neurons not only exceeds that observed during slow-wave sleep but sometimes even surpasses the level seen during wakefulness. In humans, paradoxical sleep—like that observed in animals—exhibits three key characteristics: ① low-voltage, fast-frequency brain waves; ② relaxation of neck muscle tone and suppression of spinal reflexes, resulting in strong inhibition of the motor system; ③ frequent occurrence of rapid eye movements, accompanied by large, sharp waves in certain brain structures involved in vision—including the visual cortex of the cerebral neocortex—collectively known as the pons–geniculate–occipital (PGO) waves. Since rapid eye movements occur exclusively during paradoxical sleep, the latter is often referred to as REM sleep.

　　Deep sleep

　　Generally, reduced physical activity and diminished sensory sensitivity serve as indicators for measuring sleep stages. In addition, certain physiological indicators—particularly the arousal threshold—also suggest that stages 3 and 4 of slow-wave sleep represent deep sleep phases. As for the depth of paradoxical (REM) sleep, it is much more difficult to determine, since paradoxical sleep is characterized both by muscle atonia and frequent episodes of full-body jerking and twitching of facial and finger muscles. From a sensory perspective, external stimuli that are irrelevant to the sleeper tend to be less effective in arousing him; however, when the stimulus carries special meaning or is somehow related to the content of his dreams, he will initially interpret it as something occurring within his dream. But if the stimulus is repeated several times, it can easily awaken him. These apparent contradictions suggest that during paradoxical sleep, an active process occurs within the brain that effectively disconnects the sleeper from externally irrelevant stimuli. If we assess sleep states based on the strength of autonomic nervous system activity, paradoxical sleep appears closer to the waking state: when awakened during this phase, the sleeper will report that he was deeply asleep; conversely, when awakened during slow-wave sleep, he will say that his sleep was not very sound. It is hypothesized that this subjective awareness of sleep may be closely linked to the sleeper’s dreams. In summary, accurately determining the depth of sleep remains challenging; thus, the current trend is to regard paradoxical sleep and slow-wave sleep as two distinct states.

　　Some autonomic nervous activities change as the sleep process unfolds, seemingly with little correlation to the two main circadian rhythms. For example, body temperature gradually declines from the onset of sleep, reaching its lowest point after about 5 to 6 hours, and then begins to rise again gradually. Some have suggested that people can still learn verbal materials during sleep; however, EEG analysis has shown that sleepers are actually in a state of drowsiness rather than full wakefulness. Sleep talking often occurs during stage 2 of slow-wave sleep, while sleepwalking invariably happens during stage 4 of slow-wave sleep—and in both cases, the phenomena generally have nothing to do with the content of the dreams themselves.

　　Sleep duration

　　Newborns sleep an average of 16 hours per day, and as babies grow older, their sleep duration gradually decreases, reaching about 9 to 12 hours by the age of two. Adult sleep duration varies from person to person, typically ranging from 6 to 9 hours; seven and a half hours is generally considered ideal. However, elderly individuals often sleep as little as 6 hours. According to EEG analyses, in newborns, REM sleep accounts for approximately 50% of total sleep time, and they enter the REM phase very quickly after falling asleep. In adults, REM sleep makes up about 20% of total sleep time, whereas in the elderly, it falls below 20%. For adults, sleep patterns with REM sleep accounting for less than 15% or more than 25% of total sleep time are considered abnormal. Similarly, stage 4 slow-wave sleep gradually declines with age. As for the alternation between sleep and wakefulness, newborns experience this cycle about 5 to 6 times per day; as infants grow older, the frequency gradually decreases, and school-aged children typically have only 1 to 2 such cycles per day. Some elderly individuals, however, revert to the habit of taking several naps throughout the day.

　　The default “sleep time” of the body’s internal “biological clock” is to “wake up” 6–8 hours after falling asleep; and when you “go to sleep” isn’t actually the most important factor. Moreover, as long as you’re “asleep,” the “physiological activities” that occur after you fall asleep will still take place as usual. In other words, as long as your “sleep duration” is sufficient, it won’t affect your “physical health” no matter what time of day you start sleeping. As for your “daily routine,” it’s merely a “habit”—one that can be changed at will. Of course, forming a new habit does take some time—typically 3 to 6 days—so it’s best not to switch between habits too frequently.

　　Sleep duration varies from person to person. Although the average adult needs about eight hours of sleep per day, sufficient sleep isn't simply a matter of getting more or less time—it depends on whether one can achieve a state of deep, restful sleep. Experts generally recommend that adults aim for 7 to 8 hours of sleep per night, though this can vary depending on individual differences. If you fall asleep quickly and enjoy deep, uninterrupted sleep with few or no dreams, six hours may be entirely sufficient to fully recharge your energy—and that’s perfectly fine. On the other hand, if you have trouble falling asleep, your sleep is light and shallow, and you often experience vivid or frequent dreams, even sleeping for 10 hours may fail to leave you feeling refreshed. In such cases, it’s crucial to seek various forms of treatment to achieve truly effective sleep; simply extending sleep duration alone won’t benefit your body.

　　Extend sleep

　　American scientists have found that delaying the start of school hours can help improve learning outcomes, and the later the start time, the better.

　　Biological research shows that during early childhood, people tend to go to bed and wake up early. However, as individuals enter adolescence, their circadian rhythm undergoes a significant shift, causing teenagers to stay up later and wake up later as well. This change is triggered by alterations in melatonin levels within the brain. The process typically begins around age 13, becomes markedly more pronounced between ages 15 and 16, and reaches its peak between ages 17 and 19. If school start times are adjusted to 8:35 a.m. or later, after just one semester, students’ grades in math, English, science, and social studies generally improve by about one-quarter of a grade level—for example, moving from a B to a grade somewhere between B and B+.

　　Multiple studies have shown that students at schools where class start times were shifted from 7:15 a.m. to 7:45 a.m. experienced significantly greater improvements in academic performance compared to those at schools where class start times were moved from 7:30 a.m. to 8:00 a.m. Similar findings have also been reported in studies from Brazil, Italy, and Israel. The key reason why delaying class start times yields these benefits is that teenagers are able to get at least eight hours of sufficient sleep—ideally, even nine hours would be better. By contrast, in Europe, very few middle and high schools begin classes before 9:00 a.m.

　　Scientific research

　　Sleep exhibits three key characteristics: periodicity, spontaneity, and reversibility. When measured by the animals’ state of quiet immobility and reduced sensory sensitivity, sleep behavior and corresponding changes in brain electrical activity first appear in higher vertebrates. In fish, amphibians, and reptiles, we can observe cyclical transitions between wakefulness and sleep, yet these animals do not exhibit rapid eye movement (REM) sleep. It is only in warm-blooded birds that distinct REM sleep emerges, accounting for approximately 3% to 5% of total sleep time. Among mammals—from mice to elephants—both slow-wave sleep and REM sleep are clearly present; however, the relative proportions of these two sleep stages vary significantly among different species. From the perspective of animal survival competition, we can broadly categorize mammals into two groups: one comprises prey animals, such as rodents, herbivores, and ruminants. These animals spend considerable time chewing after feeding, which shortens their overall sleep duration and reduces their REM sleep duration, typically staying below 5%. For example, cows have only 1.6% REM sleep, and anteaters even lack REM sleep altogether. The other group consists of predator animals, such as carnivores. These animals feed quickly and thus have more time available for sleep, with longer durations of REM sleep—often reaching 20% or more. Cats and dogs, for instance, fall into this category.

　　Experimentally depriving humans or animals of sleep is a viable method for studying the physiological significance and necessity of sleep.

　　After depriving someone of all sleep for 24 to 48 hours, brainwave frequencies slow down, resembling the first stage of slow-wave sleep, yet the person’s outward behavior remains normal. If sleep deprivation continues, alertness gradually declines, and in severe cases, hallucinations, delirium, or sleep talking may occur. After 3 to 4 days of continued deprivation, when the individual is finally allowed to fall asleep, the proportion of slow-wave sleep stage 4 markedly increases during the first night, while paradoxical sleep (REM sleep) correspondingly decreases. In subsequent nights, paradoxical sleep gradually compensates by increasing in duration. In one young adult who had gone without sleep for 11 days, both slow-wave sleep stage 4 and paradoxical sleep significantly increased after falling asleep. By contrast, in cats subjected to sleep deprivation, it was paradoxical sleep—not slow-wave sleep stage 4—that increased during the first night.

　　Deprive subjects of some sleep by allowing them to sleep only 3–4 hours per day. After a few days, the proportion of REM sleep—or rather, the REM component—within the first four hours of sleep tends to increase. Once the experiment is halted, subjects repeatedly enter REM sleep multiple times over several consecutive nights. In subjects who consistently sleep only 4 hours per day over an extended period, slow-wave sleep stage 4 increases, while stage 3 decreases accordingly. During the recovery phase when normal sleep patterns are resumed, there is no significant change in the slow-wave sleep stages. However, if subjects’ sleep duration is shortened to less than 3 hours per night, the development of stage 4 sleep will be impaired, severely affecting their ability to perform tasks effectively.

　　Choose to deprive sleep of a specific sleep stage. If you choose to deprive slow-wave sleep stage 4, the recovery process will compensate by increasing only stage 4 sleep. Similarly, if you choose to deprive REM sleep, an analogous compensatory response will occur. Many psychological experimental results indicate that prolonged deprivation of REM sleep does not lead to obvious psychological disturbances; however, individuals who are deprived of all sleep for extended periods cannot maintain high levels of performance over time, and their errors tend to increase significantly. This may be related to the brain’s inability to sustain a state of alertness for long periods. Furthermore, a small number of individuals may experience brief hallucinations and bizarre behaviors. In summary, these experimental findings do not seem to support the hypothesis that chronic sleeplessness or severe insomnia leads to psychopathology. On the other hand, they do suggest that prolonged sleep deprivation can easily cause fatigue, difficulty concentrating, and perceptual disturbances in vision and touch.

　　On September 17, 2014, researchers discovered a special type of neuron in the lateral region that specifically produces the neurotransmitter gamma-aminobutyric acid (GABA), which promotes deep sleep. They have developed a new tool designed to remotely control these neurons, enabling them to turn the neurons on and off for research purposes.

　　“New molecular biology techniques allow us to precisely control brain function at an unprecedented level,” said Kristelle Anserlet, a postdoctoral researcher at Harvard Medical School. “Before the development of this toolkit, we typically relied on electrical stimulation to activate target areas—but this approach often inadvertently affected neighboring regions that didn’t need stimulation, thereby complicating our research.”

　　Dolphins don’t sleep in the conventional sense; instead, they alternate between using their left and right brains to better avoid predators. The reasons animals sleep are the same as those for humans.

　　Pathological sleep

　　Insomnia

　　According to the timing of insomnia onset, three types of insomnia can be distinguished. ① Insomnia that occurs at the onset of sleep, characterized by difficulty falling asleep, is also the most common type of insomnia. ② This type is marked by frequent awakenings throughout the night, with periods of wakefulness and sleep alternating. ③ Insomnia that occurs at the end of sleep, in which patients wake up prematurely and are unable to fall back asleep. Such patients typically exhibit reduced amounts of paradoxical sleep and are prone to eliciting electroencephalographic arousal responses. From an analysis of EEG waveforms, their actual sleep duration is consistently longer than what they report. The consequences of this insomnia are generally not severe; however, long-term sufferers may occasionally experience mental fatigue, which can be effectively corrected through pharmacological treatment. In clinical practice, a combined approach using Western medicine—such as oryzanol—along with traditional Chinese medicine can achieve rapid and satisfactory therapeutic effects.

　　Hypersomnia

　　It manifests as excessive daytime sleepiness or frequent drowsiness, or as prolonged nighttime sleep. During sleep, the patient’s heart rate does not slow down, indicating that the patient does not get sufficient rest during sleep. Primary hypersomnia is often a hereditary condition associated with hypothalamic dysfunction. In addition, there is another type of hypersomnia accompanied by symptoms such as increased appetite, obesity, and inadequate breathing.

　　Narcolepsy

　　During an episode, the patient suddenly falls asleep and loses control of their actions, though the episode typically lasts only a few seconds to a few minutes. It is often accompanied by sudden collapses caused by loss of muscle tone. The EEG patterns during these episodes closely resemble those seen in REM sleep, and the condition is usually congenital. Central nervous system stimulants can help alleviate the symptoms.

　　Sleepwalking

　　This condition occurs during stages 3 and 4 of slow-wave sleep, which are also the periods when recall ability is at its lowest. It is unrelated to the patient’s dreaming. During sleepwalking, both the brain’s alertness and responsiveness are reduced, and motor coordination is impaired.

　　Enuresis

　　It typically occurs during the first one-third of sleep, with brain waves exhibiting the characteristic pattern of slow-wave stage 4. As enuresis begins, the sleep stage shifts to either stage 2 or stage 1. After being awakened, patients do not report dreaming; however, if left unawakened, they assume that an ascending inhibitory system exists within the medulla oblongata and pons, whose activity can induce sleep. This system receives sensory afferent impulses from both somatic and visceral sources on the one hand, while being under descending control from structures such as the piriform cortex of the forebrain, the cingulate gyrus, and the preoptic area on the other. At the same time, M. Jouvet proposed the monoamine theory of the wake-sleep cycle. According to his hypothesis, the ascending noradrenergic system originating from the anterior locus coeruleus maintains the electroencephalographic arousal state of the cerebral cortex; combined with the activity of the cholinergic system, this allows for the accomplishment of higher-order functions such as attention, learning, and memory. Meanwhile, the dopaminergic system of the substantia nigra-striatal circuit sustains the behavioral manifestations of wakefulness. As for the sleep process itself, the ascending serotonergic system originating from the rostral part of the raphe nuclei maintains slow-wave sleep. Once the middle part of the raphe nuclei is activated and triggers neuronal activity in the locus coeruleus, the latter's ascending impulses excite cortical electrical activity, producing high-frequency, low-amplitude waves. Simultaneously, its descending impulses inhibit the spinal motor system, thereby giving rise to paradoxical sleep. However, the authors did not specify how sleep actually initiates. It should be noted that these two hypotheses do not fundamentally conflict with each other, as their disagreement pertains only to the specific brain regions involved in sleep regulation. Today, many studies are precisely building upon these two conceptual frameworks, taking them further into greater depth. Starting in the 1970s, Monnier and J.R. Pappenheimer independently isolated peptide substances from the brains of rabbits and sheep that had just fallen asleep. When these substances were injected into the cerebral ventricles of another animal, they could induce δ waves—characteristic of slow-wave sleep. This discovery opened up new avenues for research into the mechanisms of sleep.

　　Sleep is critically important for brain health. Minors generally need more than eight hours of sleep per night, and the quality of sleep must be high. If sleep duration is insufficient or sleep quality is poor, it can endanger one’s life or have adverse effects on the brain. Brain fatigue becomes difficult to recover from, and in severe cases, it may even impair brain function. If adolescents don’t get enough sleep or their sleep quality is poor, they should appropriately increase their sleep time—for example, by taking a short nap during the summer—and should also make efforts to improve their sleep conditions.

　　According to the common view, sleep is the primary way to relieve brain fatigue. If one suffers from chronic sleep deprivation or poor sleep quality over a long period, it can severely impair brain function, causing even highly intelligent individuals to become confused and mentally foggy. Many adolescent students develop conditions such as neurasthenia, often precisely because of severe sleep deprivation.

　　Harm of Insomnia

　　Physiological experiments have shown that individuals who suffer from sleep deprivation not only experience a significant decline in immune function but also age at a rate 4 to 5 times faster than those with adequate sleep. The following section provides a systematic exploration and study of the effects of sleep on physiological tissues.

　　1. Sleep and Cellular Lifespan

　　The consequences of sleep deprivation on cellular oxidative renewal primarily lead to tissue damage. This process is repaired through the action of enzymes, and the continuous repair among molecules is inextricably linked to the overall maximum lifespan of cells. In terms of aging, the most critical task is detecting and repairing DNA damage. Moreover, the integrity of cells forms the foundation for their lifespan during division and proliferation. This repair mechanism is carried out precisely when organisms enter deep sleep.

　　Scientific experiments have shown that the duration of high-intensity sleep corresponds to the greatest effort expended by the body on average to repair DNA damage. From a cellular and molecular perspective, the balance between DNA damage and DNA repair is the most important indicator of cellular aging.

　　2. Sleep and the Body’s Immune System

　　Whether it’s insufficient sleep or a lack of adequate, restorative sleep, both can deal a devastating blow to the human immune system, causing the thymus—the body’s most important immune organ—to shrink dramatically. Under normal circumstances, the thymus typically shrinks to the size of a cherry only during middle and old age, at which point the proliferation of T cells significantly declines. As a result, not only do white blood cells and lymphocytes drastically decrease in number, but macrophages also become inhibited.

　　Without the protection of the immune system, various cancer-causing diseases pose a potentially fatal threat to the human body. Even in the short term, the body may experience nausea, dizziness, mental exhaustion, and weakness in the limbs—symptoms that not only reflect a direct decline in immune function but also serve as early warning signs of disease.

　　3. Sleep and the Nervous System

　　The nervous system is the body’s primary regulatory system, directly controlling the functions of various organs and systems within the body. The relationship between sleep and the physiological functions of the central nervous system is closely intertwined.

　　As a regulatory mechanism, sleep is mediated by the hypothalamus, which controls visceral activities and hormonal endocrine functions. Moreover, neuroendocrine regulation not only depends on instructions from the central nervous system but also reflects the intrinsic “biological clock” activity cycle of the organism itself.

　　From a physiological perspective, the biological clock is essentially a memory code encoded by DNA that governs genetic functions. For instance, in hypothalamic tissues, during deep sleep, the body typically begins secreting reducing glutathione and superoxide dismutase (SOD) around 10 p.m. each night. These are essential hormones that the body cannot synthesize on its own; they primarily serve to repair cellular damage, promote cell proliferation and differentiation, and ensure the normal growth and development of all tissues and organs.

　　4. Sleep and the Repair Mechanism

　　Experiments have shown that even short-term sleep deprivation and insufficient sleep duration can dramatically accelerate the body’s aging process and significantly weaken the immune system.

　　Deep sleep at night, mediated by the neural centers of the hypothalamus, actually regulates the metabolic activities of various tissue cells, thereby influencing the body’s physiological functions. Its primary role is to repair cellular damage and restore the integrity of tissue functions.

　　The biological clock is regulated by the suprachiasmatic nucleus (SCN) in the hypothalamus of the brain. It sends “time signals” to the central nervous system via cyclical secretion of endocrine hormones, thereby influencing the body’s circadian biological effects. This indicates that the body’s repair mechanisms must take place at night.

　　5. Sleep and Growth Hormone

　　Sleep is a biological instinct. During sleep, the body’s muscles relax completely, reducing the body’s responsiveness to external stimuli and decreasing activities such as heartbeat, breathing, and excretion—conditions that facilitate the recovery of various organs’ functions. The body’s internal biological clock regulates the endocrine system, prompting the release of various hormones. Among these hormones is growth hormone, which promotes muscle metabolism, restores physical strength, and stimulates bone growth.

　　During childhood, the secretion of this hormone follows a pattern characterized by higher levels at night than during the day—between 1 a.m. and 5 a.m., growth hormone released is nearly three times greater than during the daytime. Clearly, if infants and young children consistently stay up late, it will inevitably disrupt the normal physiological secretion of growth hormone, which can be highly detrimental to their growth and development, especially having a significant impact on height.

　　Adults who frequently stay up late tend to age more rapidly, while children exhibit emotional instability, often complain of soreness in the lower back, legs, and feet, dislike walking, experience eye fatigue easily, and some children are also prone to illnesses such as bronchitis and rhinitis.

　　Frequent late nights can also lead to constipation, and of course, this has a very significant impact on the body.

　　Therefore, to ensure the healthy growth and development of infants and young children, parents should establish regular daily routines for them, cultivate the habit of going to bed at a fixed time in the evening, and ensure they get sufficient sleep. Newborn babies’ “night crying” can affect their growth. Shortly after birth, newborns essentially don’t distinguish between day and night: they wake up to feed, and once they’re full, they fall asleep—this irregular pattern gradually becomes more regular only around 4 to 5 months of age. By 7 to 8 months, 80% of infants are awake during the day (with occasional short naps) and sleep at night; by the time they reach one year old, their daily routine closely resembles that of adults.

　　6. The impact of creative thinking on the brain

　　Researchers believe that for the human brain to think clearly and respond swiftly, it must get sufficient sleep. If one suffers from chronic sleep deprivation and the brain doesn't get adequate rest, it can impair the brain's creative thinking and its ability to handle tasks effectively.

　　7. Affects the growth and development of adolescents

　　In addition to factors such as genetics, nutrition, and exercise, adolescents’ growth and development are also closely related to the secretion of growth hormone. Growth hormone is a hormone secreted by the hypothalamus, and it promotes the development of bones, muscles, and organs. Since growth hormone secretion is closely linked to sleep—specifically, there’s a major secretion peak after a person falls into deep sleep, followed by several smaller peaks—and since growth hormone secretion decreases when one is not asleep, it’s crucial for adolescents to get sufficient sleep if they want to grow well and reach their full height potential.

　　8. Affects skin health

　　The reason human skin remains soft and radiant is that the capillaries in the subcutaneous tissue provide it with ample nutrients. Lack of sleep can cause congestion in the skin’s capillaries, obstructing circulation and depriving skin cells of sufficient nourishment. This, in turn, disrupts the skin’s metabolism, accelerates aging, and makes the skin appear dull and pale. In particular, dark circles under the eyes become more pronounced, and wrinkles tend to form more easily.

　　9. Leading to the onset of disease

　　Frequent sleep deprivation can lead to feelings of worry and anxiety, weaken the immune system, and consequently increase the risk of various illnesses, such as neurasthenia, colds, and gastrointestinal disorders. Lack of sleep can also cause elevated cholesterol levels in the blood, thereby increasing the likelihood of heart disease. Moreover, cell division in the human body primarily occurs during sleep; insufficient sleep or disrupted sleep patterns can interfere with normal cell division, potentially triggering mutations that give rise to cancer cells and ultimately leading to cancer. Generally speaking, people of different age groups have varying daily sleep requirements: middle school students should aim for 8–9 hours of sleep per day, while adults need 7–8 hours of sleep each day.

　　10. Sleep deprivation can lead to obesity.

　　Relevant studies have shown that sleep deprivation can lead to a decrease in the concentration of adiponectin in the human body. Adiponectin is a substance active in the circulatory system, with the function of suppressing appetite and influencing the brain’s decision-making process regarding whether or not to eat. At the same time, sleep deprivation can also cause an increase in the concentration of appetite-stimulating hormones in the body. These appetite-stimulating hormones are secreted by the stomach and can trigger a person’s desire to eat. When these key regulatory systems in the body—responsible for controlling appetite—begin to conflict with each other, the brain’s decision-making system may end up making incorrect decisions.

　　If people can maintain regular sleep schedules, they may prevent disruptions to their body’s appetite-regulating mechanisms, thereby keeping their weight within a relatively normal range.