Sleep is the time when your body and brain rest and recharge. When you sleep, your eyes close and your body gets very still and calm. But your brain does not turn off. It keeps doing quiet, important work all night long. People spend about one third of their whole lives asleep, so sleep is a big part of being a person.
A dream is a story or set of pictures that your mind makes while you sleep. Almost everyone dreams every night, even people who never remember a single dream in the morning. Some dreams are fun, some are silly, and some are a little scary. They all happen inside your head while your body rests.
Why sleep and dreams are tricky to understand
A lot of people think your brain shuts off when you fall asleep, like a light switch going off. That is not true. Your brain stays busy all night. One of its big jobs is sorting through everything you learned that day and storing the important parts so you can remember them. That is why a good night of sleep can help you do better on a test.
Another tricky thing is dreaming. Many people think you can only dream during one special part of sleep. You do have your most colorful, movie-like dreams in a stage called REM sleep. But scientists have learned that quieter dreams can happen in other parts of sleep too.
People also mix up sleepwalking and dreaming. It seems like a sleepwalker must be acting out a dream. But sleepwalking happens during deep sleep, not during the dreaming stage. During the dreaming stage, your body holds very still instead.
Key facts about sleep and dreams
Sleep takes up about a third of your whole life. Sleep is just as important to your body as food and water.
Your brain stays busy while you sleep. It does not switch off. One job is to help you remember what you learned.
Sleep happens in stages. You move from light sleep into deep sleep and then into dream sleep, over and over all night.
One trip through the stages takes about an hour and a half. You repeat it four to six times a night.
REM sleep is the dream stage. REM stands for rapid eye movement, because your eyes dart around behind your closed eyelids.
Almost everyone dreams. Some people just do not remember their dreams when they wake up.
Your body stays still while you dream. This keeps you from getting up and acting out your dreams.
Newborn babies sleep the most. They sleep about 14 to 17 hours a day because they are growing so fast.
Grown-ups need about 7 or more hours of sleep. Kids and teens need even more than grown-ups do.
Darkness helps you feel sleepy. When it gets dark, your brain makes more of a sleep helper called melatonin.
Common myths about sleep and dreams
Myth: Your brain turns off when you sleep. Your brain stays active all night. It sorts your memories, helps your body rest, and does other quiet work while you sleep.
Myth: Only some people dream. Almost everyone dreams every single night. People who say they never dream usually just forget their dreams by morning.
Myth: Sleepwalkers are acting out their dreams. Sleepwalking happens during deep sleep, not during the dream stage. While you dream, your body stays still and relaxed.
Myth: You can train yourself to stop needing sleep. Everybody needs sleep, at every age. Going without sleep makes it hard to think clearly and feel good.
Myth: A scary dream means something bad will really happen. A dream is just your mind making pictures while you sleep. Dreams do not come true or tell the future.
Frequently asked questions about sleep and dreams
Why do we sleep?
Sleep gives your body and brain a chance to rest and repair. While you sleep, your brain stores memories and your body grows and heals. Without enough sleep, it is hard to pay attention, learn, and feel happy. Sleep is so important that you spend about a third of your life doing it.
Why do we dream?
Scientists are still working out exactly why we dream. Dreams seem to be tied to the busy work your brain does at night, like sorting memories and feelings. Your most vivid dreams happen during REM sleep, when your brain is very active. Almost everyone dreams, even if they do not remember it.
How much sleep do kids need?
Kids need more sleep than grown-ups. School-age kids do best with about 9 to 12 hours of sleep each night. Teenagers need about 8 to 10 hours. Grown-ups need about 7 to 9 hours. Getting enough sleep helps you learn, grow, and feel your best.
What is a nightmare?
A nightmare is a scary or upsetting dream. Like most vivid dreams, nightmares happen during REM sleep. Almost everyone has a nightmare now and then, and they are very common when you are a kid. A nightmare is only a dream, so it cannot hurt you.
Why does it feel easier to fall asleep when it is dark?
When the light around you fades, your brain makes more of a sleep helper called melatonin. Melatonin sends a signal that it is time to feel sleepy. Bright lights and glowing screens at night can keep that signal from working, which can make it harder to fall asleep.
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Sleep is a natural state of rest in which your body relaxes and your awareness of the outside world drops, while your brain stays active and does important behind-the-scenes work. It is not one long, even rest. Your brain moves through different stages of sleep, again and again, all night long. People spend about one third of their lives asleep, which is roughly as much time as they spend awake and busy during the day.
A dream is a series of images, feelings, and events that your mind creates while you sleep. Your most vivid, story-like dreams happen during a stage called REM sleep, when your brain is almost as active as when you are awake. Nearly everyone dreams, even people who never seem to remember a dream. Forgetting a dream does not mean it never happened.
Why sleep and dreams are tricky to understand
People often picture sleep as a single, flat state, like a phone that is simply switched off. Real sleep is more like a journey through stages. You start in light sleep, sink into deep sleep, and then enter REM sleep, where most dreaming happens. One full trip through these stages is called a sleep cycle, and it takes about 90 minutes. You repeat the cycle four to six times each night.
Dreaming is another source of confusion. For a long time, scientists believed dreams happened only during REM sleep. They now know that dreaming can occur during non-REM sleep too. The REM dreams are usually the wildest and most vivid, while non-REM dreams are often calmer and more plain.
Sleepwalking trips people up as well. It looks like a sleepwalker must be acting out a dream, but the opposite is true. Sleepwalking happens during deep non-REM sleep, mostly in the first part of the night. During REM sleep, the stage with the most dreaming, your body’s muscles relax and stay still, so you cannot get up and move around then.
Key facts about sleep and dreams
Sleep happens in stages. You move from light non-REM sleep into deep non-REM sleep, then into REM sleep, over and over.
One sleep cycle lasts about 90 minutes. Most people complete four to six cycles per night.
REM stands for rapid eye movement. During REM, your eyes dart around under closed eyelids and your brain is highly active.
REM is the main dream stage. Your most vivid, movie-like dreams happen here, though dreaming can occur in other stages too.
You get most of your deep sleep early in the night. As the night goes on, deep sleep shrinks and REM grows.
Your body stays still during REM. This is your brain’s way of keeping you from acting out your dreams.
Your body runs on a roughly 24-hour clock. Scientists call this the circadian rhythm, and it controls when you feel sleepy or awake.
Darkness raises melatonin. Your body makes more of this hormone when light fades, which helps you feel sleepy.
Teenagers need 8 to 10 hours of sleep. That is more than the 7 to 9 hours most adults need.
Sleep helps memory. While you sleep, your brain strengthens and stores what you learned during the day.
Common myths about sleep and dreams
Myth: We only dream during REM sleep. Dreaming also happens during non-REM sleep. REM dreams are simply more vivid and story-like than the dreams in other stages.
Myth: Sleepwalkers are acting out a dream. Sleepwalking happens during deep non-REM sleep, not during REM. In REM, the body is relaxed and still, so a person cannot walk around then.
Myth: A nightmare and a night terror are the same thing. They are different. A nightmare is a vivid dream during REM sleep, and people often remember it. A night terror comes out of deep non-REM sleep and is usually not remembered.
Myth: Bright sunshine makes you sleepy. Bright light actually lowers melatonin and signals your body to stay awake. Darkness is what raises melatonin and helps you feel sleepy.
Myth: Pulling an all-nighter before a test is a smart move. Sleep helps your brain store what you studied. Skipping sleep usually makes your memory and focus worse, not better.
Frequently asked questions about sleep and dreams
What is REM sleep?
REM sleep is the stage of sleep tied to your most vivid dreams. The letters stand for rapid eye movement, because your eyes move quickly under your closed eyelids during it. In REM, your brain is highly active, almost like being awake, while your body’s muscles stay relaxed. You enter REM several times each night, and the REM periods get longer toward morning.
Why do we dream?
Scientists are still studying exactly why we dream. Dreams appear to be connected to the work your brain does at night, such as sorting memories and processing feelings. Most vivid dreams happen during REM sleep, but quieter dreams can occur in other stages too. Whatever their purpose, dreams are a normal part of healthy sleep.
How much sleep do I need as a tween or teen?
Tweens and teens need more sleep than adults. Kids ages 6 to 12 do best with about 9 to 12 hours each night, and teenagers ages 13 to 18 need about 8 to 10 hours. Many teens struggle to get that much because of early school start times and busy schedules. Getting enough sleep helps with focus, mood, learning, and growing.
What is a body clock?
Your body clock is an inner timer that runs on a roughly 24-hour rhythm, called the circadian rhythm. It helps decide when you feel awake during the day and sleepy at night. Light is the main signal that keeps this clock set to the day-night cycle. When evening comes, your clock tells your brain to make more melatonin so you start to feel sleepy.
Does sleep really help me remember things?
Yes. While you sleep, your brain replays and strengthens the things you learned that day, moving important memories into longer-term storage. This is one reason studying and then getting a full night of sleep can help more than staying up late to cram. A well-rested brain remembers and focuses better than a tired one.
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Sleep is a reversible state of reduced awareness and responsiveness in which the body rests while the brain runs through an organized sequence of activity. It divides into two broad types: non-REM sleep, made up of three stages, and REM sleep, named for the rapid eye movements that mark it. A person passes through these stages in cycles of roughly 90 minutes, four to six times per night, and spends about one third of life asleep. A dream is the experience of images, sensations, and events generated by the mind during sleep, most vivid during REM but possible in other stages too.
What is often misunderstood about sleep and dreams
Sleep is easy to picture as a single passive state, a kind of nightly off switch. The reality is closer to a structured cycle. Each cycle runs from light non-REM sleep (stage N1), into a deeper non-REM stage (N2), into the deepest non-REM stage (N3, also called slow-wave sleep), and then into REM sleep. The mix shifts across the night. Deep slow-wave sleep dominates the early cycles, while REM periods lengthen toward morning, so the longest, most vivid dreaming tends to happen in the hours before you wake.
The link between REM and dreaming is real but often overstated. REM sleep is the stage most strongly tied to vivid, narrative dreams, and the brain during REM shows electrical activity close to that of waking, which is why REM is sometimes called paradoxical sleep. Yet dreaming is not exclusive to REM. Research has established that dreaming also occurs during non-REM sleep, where dream reports tend to be calmer and less story-like. The old shorthand that REM equals dreaming is no longer accurate.
A separate misconception surrounds sleepwalking and night terrors. Both look like dream enactment, but neither happens during REM. They are non-REM events that arise out of deep slow-wave sleep, usually in the first part of the night, and they are grouped together as disorders of arousal. During REM, by contrast, the body enters a near-total loss of muscle tone called atonia. This paralysis spares the muscles used for breathing and the eyes, but it keeps the limbs still, which is thought to prevent a sleeper from physically acting out a dream. So the stage with the most vivid dreams is also the stage when the body is least able to move.
Finally, the idea that the brain switches off during sleep gets the biology backward. Sleep is an active process. The brain consolidates memories, regulates hormones, and clears waste products while you rest. Skipping sleep does not save time so much as borrow against your ability to think, remember, and stay healthy.
Key facts about sleep and dreams
Two types, four stages. Sleep splits into non-REM sleep (stages N1, N2, and N3) and REM sleep. N3 is the deepest non-REM stage and the hardest to wake from.
About one third of life. People spend roughly a third of their lives asleep, which makes sleep as fundamental to survival as food and water.
The 90-minute cycle. A full cycle through the stages lasts about 90 minutes, and most people complete four to six cycles per night.
NREM is the majority. Non-REM sleep accounts for about 75 to 80 percent of total sleep in adults, with REM making up the remaining fifth to quarter.
REM is paradoxical. During REM, brain activity resembles wakefulness, the eyes move rapidly, and the most vivid dreaming occurs, all while the body is paralyzed by atonia.
Dreaming is not REM-only. Vivid, narrative dreams cluster in REM, but quieter dreams also occur during non-REM sleep.
Deep sleep comes first. Slow-wave sleep is concentrated in the early cycles, while REM periods grow longer as the night progresses.
The body clock runs near 24 hours. The circadian rhythm, governed by a master clock in the brain, is set close to the 24-hour day by light.
Melatonin rises in darkness. As light fades, the brain increases melatonin, a hormone that signals the onset of sleepiness.
Sleep needs change with age. Newborns need about 14 to 17 hours a day, teenagers about 8 to 10, and adults 7 or more.
Sleep supports memory. During sleep, the brain replays and strengthens recent learning, which is why rest before a test outperforms an all-nighter.
The record is a warning, not a goal. In 1964, teenager Randy Gardner stayed awake about 264 hours, roughly 11 days, which left him struggling to think and concentrate.
Common myths about sleep and dreams
Myth: We only dream during REM sleep. REM is the stage of the most vivid, story-like dreams, but dreaming also occurs during non-REM sleep. Non-REM dreams are usually calmer and less narrative.
Myth: The brain shuts down during sleep. Sleep is active. The brain consolidates memory, regulates hormones, and clears waste while the body rests. Activity changes character across stages rather than stopping.
Myth: Sleepwalking is acting out a dream. Sleepwalking and night terrors arise from deep non-REM sleep, not from REM. During REM, atonia keeps the body still, so dream enactment is normally blocked.
Myth: Everyone needs exactly eight hours. Sleep needs vary by age and person. Adults generally need 7 to 9 hours, teenagers 8 to 10, and younger children more. The right amount is the one that leaves you rested.
Myth: You can train your body to need less sleep. Long-term sleep loss reliably harms attention, memory, mood, and health. People do not adapt away the need for sleep, even if they get used to feeling tired.
Myth: A nightmare predicts the future. A nightmare is a vivid, distressing dream that occurs during REM sleep. It reflects the mind’s nighttime processing, not events to come.
Frequently asked questions about sleep and dreams
What are the stages of sleep?
Sleep has four stages across two types. Non-REM sleep includes stage N1, the brief transition into sleep; stage N2, a period of lighter sleep that makes up much of the night; and stage N3, the deepest non-REM stage, known as slow-wave sleep. The fourth stage is REM sleep, marked by rapid eye movements, a highly active brain, and vivid dreaming. A typical cycle moves from N1 through N3 and into REM, then repeats about every 90 minutes.
What is REM sleep, and why is it called paradoxical?
REM sleep is the stage tied to the most vivid dreaming. It is called paradoxical sleep because the brain’s electrical activity looks much like that of an awake brain, even though the sleeper is unconscious and the body’s skeletal muscles are paralyzed. The eyes move rapidly under closed lids, and breathing and heart rate become more variable. REM periods recur several times a night and lengthen toward morning.
Why does the body stay still during dreams?
During REM sleep, the body enters a state of muscle paralysis called atonia. The muscles needed for breathing and eye movement keep working, but the limbs go slack. Researchers think this paralysis protects the sleeper by preventing the physical enactment of dreams. When this system does not work properly, a person may move in ways that match the dream, which is the basis of REM sleep behavior disorder.
How much sleep do people need?
Sleep needs depend on age. Newborns need about 14 to 17 hours per day, school-age children about 9 to 11 hours, teenagers about 8 to 10 hours, and adults 7 or more hours per night. Older adults need a similar amount to younger adults, around 7 to 8 hours. Consistently getting enough sleep supports memory, mood, immune function, and overall health.
What controls when we feel sleepy?
Two main forces shape sleepiness. The first is the circadian rhythm, a roughly 24-hour internal clock run by a master timekeeper in the brain and set by light. The second is sleep pressure, which builds the longer you stay awake. As evening light fades, the body releases more melatonin, a hormone that signals it is time to wind down. Bright light at night, including from screens, can blunt that signal and delay sleep.
How long can a person go without sleep?
In 1964, 17-year-old Randy Gardner stayed awake about 264 hours, or roughly 11 days, as a science-fair project, setting a record at the time. As the days went on, he had clear trouble with concentration, mood, and thinking. The experiment is a useful illustration of how badly the brain works without sleep, and it is not something to copy. Prolonged sleep loss is unsafe.
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Sleep is a reversible behavioral state defined by reduced responsiveness to the environment and a characteristic, cyclic progression of brain activity, divided into non-REM and REM sleep and measured by electrical recordings of the brain, eyes, and muscles. Across a night, a person alternates between non-REM stages, which carry the body’s deepest rest, and REM sleep, which carries its most active brain states and most vivid dreaming. A dream is the subjective experience produced during sleep, richest and most narrative during REM, and the night’s architecture, hormones, and waste-clearance systems are all tuned to this NREM-REM alternation. The whole cycle repeats roughly every 90 minutes, four to six times a night.
What is often misunderstood about sleep and dreams
The deepest misconception is that sleep is a uniform, passive shutdown. Continuous recording with electroencephalography (EEG) shows the opposite: an organized sequence of distinct states, each with its own electrical signature. Stage N1 is the light transition into sleep. Stage N2 is identified by two hallmark features, sleep spindles (short bursts of fast, waxing-and-waning activity) and K-complexes (large, sharp single waves), and it occupies roughly half of a typical night. Stage N3, slow-wave sleep, is dominated by large, slow delta waves and is the hardest stage to be woken from. REM sleep then breaks the pattern entirely, with low-amplitude, fast EEG activity that resembles waking. This is why REM earned the label paradoxical sleep: a brain that looks awake paired with a body that cannot move.
REM and dreaming are tightly associated but not identical. Vivid, narrative dreams concentrate in REM, yet decades of dream-report research confirm that dreaming also occurs in non-REM sleep, typically with more fragmentary or thought-like content. Treating REM as synonymous with dreaming, a common simplification, understates how distributed dreaming actually is across the sleep period.
REM is also where the boundary between brain and body is most striking. During REM, descending pathways actively inhibit the motor neurons that drive skeletal muscle, producing a near-complete loss of muscle tone known as REM atonia. The diaphragm and the muscles that move the eyes are spared, which is why breathing continues and the eyes dart, but the limbs are effectively paralyzed. This is normally protective: it prevents the enactment of dream content. When the atonia system fails, dreams can be acted out, which is the defining feature of REM sleep behavior disorder. The everyday assumption that sleepwalking is dream enactment inverts the biology, because sleepwalking and night terrors arise from deep slow-wave sleep, when dreaming is sparse and atonia is absent, not from REM.
A final misunderstanding concerns what sleep is for. Far from idling, the sleeping brain performs measurable maintenance. It consolidates memory, releases hormones in stage-dependent patterns, and supports the clearance of metabolic waste. The cost of skipping sleep is not saved time but degraded cognition and physiology, a trade the brain cannot make indefinitely.
Key facts about sleep and dreams
Sleep architecture has a fixed grammar. A cycle runs N1, N2, N3, then REM, and repeats about every 90 minutes, with four to six cycles per night. Slow-wave sleep loads the early cycles and REM loads the later ones.
N2 has signature waveforms. Sleep spindles and K-complexes define stage N2, which makes up roughly half of total sleep time. K-complexes are among the largest events in the healthy human EEG.
NREM is the majority share. Non-REM sleep accounts for about 75 to 80 percent of an adult night; REM occupies about 20 to 25 percent.
REM is paradoxical and paralyzed. REM combines waking-like EEG activity, rapid eye movements, and vivid dreaming with active muscle atonia that spares respiration and the eyes.
Infants are REM-rich. Newborns spend a far larger share of sleep in REM than adults, on the order of half, a pattern thought to support the rapid development of the brain.
The master clock is the SCN. The suprachiasmatic nucleus, a cluster of neurons in the hypothalamus, is the body’s master circadian clock, reset daily by light reaching the retina along the retinohypothalamic tract.
The intrinsic clock runs slightly long. Studies of people isolated from time cues place the human circadian period near 24.2 hours on average, so daily light is needed to align it to the 24-hour day.
Melatonin is the darkness signal. The SCN drives melatonin release from the pineal gland as light falls, raising the propensity for sleep.
Two processes govern timing. Sleepiness reflects the interaction of the circadian clock and a homeostatic sleep pressure that rises with time awake; adenosine accumulation is one molecular driver of that pressure, and caffeine acts by blocking adenosine receptors.
The glymphatic system and sleep. A 2013 mouse study found that clearance of metabolic waste through cerebrospinal fluid speeds up markedly during sleep, alongside an expansion of the space between brain cells, though a 2024 study reported reduced clearance during sleep, so whether clearance rises during sleep is now debated.
Slow-wave sleep aids consolidation and growth. Deep slow-wave sleep is associated with the transfer of recent memories from the hippocampus toward the neocortex, and much of the body’s growth hormone is released during this stage.
Some animals sleep one hemisphere at a time. Dolphins and certain other species use unihemispheric slow-wave sleep, resting one cerebral hemisphere while the other stays alert enough to surface and breathe.
Common myths about sleep and dreams
Myth: We dream only during REM sleep. REM hosts the most vivid, narrative dreams, but dreaming is also documented in non-REM sleep, usually with more fragmentary content. The REM-equals-dreaming shorthand is outdated.
Myth: The brain switches off in sleep. Sleep is metabolically and electrically active. It consolidates memory, releases hormones in stage-specific patterns, and clears metabolic waste through the glymphatic system.
Myth: Sleepwalking and night terrors are REM dream enactment. They are disorders of arousal from deep slow-wave sleep, occurring mostly in the first part of the night. REM atonia normally prevents movement during REM, so true dream enactment is the exception, seen in REM sleep behavior disorder.
Myth: The human body clock is exactly 24 hours. The intrinsic circadian period averages closer to 24.2 hours. Because it is slightly longer than the solar day, the clock would drift later without daily resetting by light.
Myth: Caffeine adds energy. Caffeine does not supply energy or remove adenosine. It blocks adenosine receptors, masking the sleep pressure that adenosine signals, which is why tiredness can rebound as the caffeine wears off.
Myth: You can train yourself out of needing sleep. Chronic sleep restriction degrades attention, memory, mood, and physiological health. Tolerance to feeling tired is not the same as eliminating the underlying need.
Frequently asked questions about sleep and dreams
What defines each stage of sleep on an EEG?
Each stage has a recognizable electrical pattern. Stage N1 shows a drop from the relaxed-waking rhythm into low-amplitude, mixed-frequency activity. Stage N2 is marked by sleep spindles and K-complexes. Stage N3, slow-wave sleep, is dominated by high-amplitude, slow delta waves and is the deepest non-REM stage. REM sleep returns to low-amplitude, fast activity that resembles waking, accompanied by rapid eye movements and muscle atonia. Sleep technologists score these features to map a night’s architecture.
How does the body clock keep time, and what role does light play?
The master circadian clock is the suprachiasmatic nucleus, a small group of neurons in the hypothalamus positioned just above where the optic nerves cross. Specialized light-sensing cells in the retina send timing signals to it through the retinohypothalamic tract, which is why light is the dominant cue that keeps the clock aligned to the 24-hour day. The clock’s own intrinsic period, measured when people are isolated from time cues, averages close to 24.2 hours, so without daily light it tends to drift. The clock also drives melatonin release from the pineal gland as evening light falls.
What is the glymphatic system, and why does it matter for sleep?
The glymphatic system is a brain-wide network that circulates cerebrospinal fluid through brain tissue to carry away metabolic waste. In a 2013 study, researchers found that this clearance increases sharply during sleep in mice, accompanied by an expansion of the space between brain cells that lets fluid flow more freely. The finding offered a concrete reason that sleep supports brain health: the brain uses the resting state to flush out byproducts that build up during waking activity. A 2024 study, however, reported reduced brain clearance during sleep, so whether sleep increases clearance, and how directly these animal findings extend to humans, is now actively debated.
How do adenosine and the two-process model explain sleepiness?
Sleep timing is often described with a two-process model. One process is the circadian rhythm, the roughly 24-hour clock that promotes wakefulness during the day and sleep at night. The other is homeostatic sleep pressure, which accumulates the longer you stay awake and dissipates during sleep. Adenosine, a molecule that builds up in the brain as energy is used during waking hours, is one driver of that pressure, and its buildup is associated with rising sleepiness. Caffeine produces alertness by blocking adenosine receptors rather than by removing adenosine, so the underlying pressure remains and can resurface as the caffeine clears.
Why do newborns spend so much time in REM, and how does that change with age?
In adults, REM accounts for about 20 to 25 percent of total sleep. Newborns spend a far larger share, on the order of half their sleep, in REM. The high proportion of REM in early life is widely thought to support the rapid wiring of the developing brain, with the internally generated activity of REM helping to shape neural connections. The REM share declines through infancy and childhood toward adult levels as the brain matures.
Do all animals sleep the same way?
No. Sleep is widespread across the animal kingdom, but its form varies. Dolphins and some other cetaceans, along with certain birds and seals, use unihemispheric slow-wave sleep, in which one cerebral hemisphere enters slow-wave sleep while the other remains alert. For a dolphin, this allows continuous swimming, periodic surfacing to breathe, and vigilance for threats, typically with the eye opposite the awake hemisphere kept open. The adaptation shows that the deep rest of slow-wave sleep can, in some species, be split between the two halves of the brain.
