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Sleep: Facts

Man yaning in bed with arms outstretched

Everyone sleeps! In fact, in terms of hours spent sleeping, we spend almost one-third of our lives sleeping. Nearly every night of our life, our bodies go into a state of relaxation where we are not aware of what's happening around us. During sleep, our eyes are closed, our breathing is regular, and we do not respond to sound or light. For most of history, people believed that sleep was a passive, inactive state. But we are not doing “nothing!” Now we know that even while we sleep, our brains are highly active, doing things that help us become healthier and stronger. Let's learn more about sleep.

Why Do We Need Sleep?

We have all known times when we are really, really tired. We feel like we just can't stay awake any longer, and we want to go to bed more than anything else. We also know that after a good night's sleep we feel better, have more energy and can deal with daily challenges more effectively. Although sleep is essential to our well-being, scientists don't really know why we sleep or even what sleep actually is. They continue to study sleep and to learn about the important things that happen to our bodies when we get enough sleep, and the negative consequences when we don't.

Boy and girl measuring their heights

One reason we need to sleep is that it makes our bodies healthier. Sleep restores the energy we need to accomplish all the working and playing we do each day. Also, sleep is when most growth hormones are released for the growth and development of muscles, bones and brain cells, which is particularly important for young bodies. Sleep is also important to the body's ability to rejuvenate and make repairs. During sleep, the body repairs cells which allows for healing from injuries and illness. That is why doctors tell people who are sick to get lots of rest! Sleep is also important for maintaining a healthy immune system to ward off infection. People who lack sleep are more likely to become sick. In adults, poor sleep is associated with illnesses such as diabetes, obesity, dementia, depression and heart disease.

Another benefit of sleep is that it helps us to regulate moods and emotions. Have you ever gone to sleep feeling upset about something, but when you woke up in the morning you felt better, and the problem didn't seem so big? The lack of a good night's sleep can cause you to feel cranky and to have a harder time dealing with problems during the day. In fact, the first segment of the brain that begins to suffer when we don't get enough sleep is the prefrontal cortex, the center of decision-making and problem-solving. Without enough sleep, you may cry more easily or feel overwhelmed by everyday challenges.

Child sleeping on books

An important reason we need sleep is that sleep has a strong impact on learning and memory. When you're awake, you are busy taking in information and performing tasks that give your body and brain a real workout. When you sleep, your brain goes into “housekeeper” mode and sorts out all those activities from the day. New memories are consolidated during sleep so that you can remember the things you learned. This seems to be true both for learning information and for learning procedures to perform tasks. During sleep, your brain clears out the clutter and retains the important things, almost like a “save” button. Then in the morning, your brain is once again ready to collect new information.

Sleep is also related to safety. Without sufficient sleep, people tend to have slower reaction times. Drivers have more car accidents, and workers may make more errors as their focus and judgment are impaired. Sleep-related accidents and errors are responsible for many serious injuries, deaths, and economic losses. It is estimated that each year in the United States, about 100,000 traffic accidents are due to drowsy driving. When we sleep well, we are more able to concentrate, follow directions and pay attention.

One other nice benefit of sleep is that we tend to look our best when we are rested!

How We Sleep

Person sleeping and waking up

How and when you feel sleepy has to do with chemicals in the brain. An area of the brain called the SCN is especially sensitive to signals of light and dark. Even when you are sleeping, your brain can detect light through the thin skin of your eyelids. Morning light causes the SCN to trigger the release of hormones to help you wake up and feel alert. But when the sun goes down, your brain switches to night mode. With the coming of darkness, the SCN sends a message to the pineal gland located in the center of your brain. This gland is stimulated to send out a hormone called melatonin. Melatonin causes your body to relax and to feel ready for sleep.

When we fall asleep, our muscles relax, our temperature and blood pressure drop slightly, our breathing rate is regular, and our heart beats more slowly. A sleeping person is unconscious to most things happening in the environment. However, a sleeping person can be woken if the stimulus is strong enough. If you shake the person or yell loudly, they will wake up.

Body Clocks

Clock with a smile

All of us have two different internal clock systems that drive our sleep times and our awake times. These internal clocks work together to keep our bodies and brains in sync with the sun, tracking light and darkness.

Our bodies have one clock that says we should sleep after being awake for a long period of time. For most people, our wakeful period is during the daylight hours because humans are not well equipped for seeing in the dark; daytime is when we can best see to do our work and play. This wakeful period usually lasts 14-16 hours. After this long stretch of wakefulness, we then need sleep. By this time, it is usually nighttime.This is called sleep/wake homeostasis or sleep/wake balance.

Older man asleep on a chair with small child near him

Our other internal clock is known as circadian rhythm. The word “circadian” means that it occurs in a 24-hour cycle. This rhythm causes a series of wakeful times followed by times that we are likely to be drowsy. We feel sleepy or alert at regular times each day. In humans, the sleep drive is strongest between the hours of 1:00-5:00 am, and a milder drowsy time is often between 1:00-3:00 in the afternoon. These hours of needing sleep vary by a person's age, level of activity, foods eaten and other personal behaviors and routines. Teenagers often experience a shift in their circadian rhythm that tells them to stay up later at night and sleep longer in the morning. Grandparents' circadian rhythms also shift as they start to get sleepy earlier in the evening.

Stages Of Sleep

Man Sleeping in Bed

Our sleep may seem like one big long night of snoozing, but the truth is, there are several different kinds of sleep that take place. Our sleep is divided up into distinct sleep cycles of about 90 minutes each. There are five stages in each sleep cycle, and the sleep cycle is repeated five or six times during the night. Scientists are able to learn about sleep stages by measuring people's brain waves while they sleep. Each stage of sleep has distinct qualities and purpose. As we cycle through the several stages of sleep, our bodies heal, our energy levels are restored, our memories consolidate, and our moods are regulated.

In stage 1, we are between being awake and asleep. This is very light sleep where our muscle activity slows down and we begin to “drift off,” but we may still be partially aware of things going on around us. In Stage 2, our body temperature drops, our heart rate slows, and our breathing becomes regular. We are unaware of the surrounding environment. You experience Stage 2 sleep when you fall asleep during a movie or TV show and then are surprised to find that you missed important parts of the story. Stage 2 can last up to 50 minutes during the night's first 90-minute sleep cycle.

By the time we reach stage 3 and 4, blood pressure has dropped and we are fully relaxed. This is the deepest part of sleep, and it can be difficult to wake someone who is in this sleep stage. During deep sleep, our brainwave patterns slow down. We do not dream, move, or feel pain during stage 4. We can't even shiver! This deep coma-like sleep allows the body to heal and repair, and it is during this time that essential growth hormones are released.

man in side profile with waves drawn over the head

Stage 5 is known as REM (Rapid Eye Movement) sleep. During REM sleep, our eyes actually move back and forth behind our closed eyelids. Muscles become temporarily paralyzed in this sleep phase, and our breathing and heart rates quicken. Almost all dreaming occurs during this stage. Although we are sound asleep, our brains are fully active and brain waves appear much like they do when we are awake. During REM sleep, our brains reorganize information that we have received during the previous day, and things we learned are committed to long-term memory.

During sleep stages 1-4, we are said to be in NREM sleep (Non Rapid Eye Movement.) We spend about 80% of our sleeping hours in NREM sleep and about 20% of the night in REM sleep. Sleep scientists believe that these specific sequences of NREM and REM sleep are essential for our physical and mental rejuvenation. People who don't get sufficient sleep in either NREM or REM stages have poorer health.

How Much Sleep Do We Need?

Child sleeping

The amount of sleep a person needs can depend on many things. First of all, the amount of sleep needed depends on a person's age. The younger we are, the more sleep we require. Newborn babies spend most of their day asleep. They can sleep anywhere between 14 to 17 hours per day. As we grow older, we need fewer hours of sleep each day. Preschool children need 10-13 hours of sleep each day. School aged children need 9-11 hours of sleep per day, while adults need 7-9 hours.

student sleeping at desk

Teenagers are an interesting group when it comes to sleep. Teens need 8-10 hours of sleep each night, but they often need this sleep during a different block of time than most other people. Research shows that in teenagers, the body releases melatonin later at night than in children and adults. Thus, their internal sleep clock resets and they naturally fall asleep later at night and wake up later in the morning. This can interfere with school schedules, sports, homework, and after-school activities or jobs. Because of this, they are often sleep deprived.

How much sleep a person needs also depends on their health, how much energy they expend each day, and any sleep problems they may have. For example, you need more sleep when you have a cold so that your body can fight off the illness.

Take a look at this chart from The National Sleep Foundation to see how much sleep you should be getting. A sleep calculator can help you figure out how to get the right amount of sleep each night.

Sleep Deprivation

We are healthier when we follow our internal clocks, but circadian rhythm can be disrupted by several factors, leading to sleep deprivation. Sleep deprivation is a serious problem in our modern world. The US Centers for Disease Control and Prevention estimates that more than 80 million Americans are chronically sleep deprived, which means they regularly get less sleep than the recommended amount. The average American today sleeps about 1½ - 2 hours less than a century ago.

person looking at phone in bed

One common reason for sleep deprivation is the widespread use of artificial lights and electronic devices such as televisions, computers, tablets and smartphones. Not only can watching an exciting movie or playing a video game before bed make it hard to relax, but the actual light from electronic screens is also a problem. Cellphones, tablets, televisions, and computers all feature screens that emit blue light, which can interfere with the circadian rhythm that guides our sleep cycle, especially when we are exposed to it at night and up close. Blue light signals our internal clocks that it's daytime even when it is late at night, and suppresses melatonin release, making it harder for us to fall asleep. This disruption to our internal clocks can lead to reduced energy levels, irritability, and poor judgment.

Earth with numbers around it

Another cause of sleep deprivation is “jet lag.” Jet lag creates a disruption in our body clocks and circadian rhythms. If someone travels a long distance by plane, they may be moving rapidly to a time zone that is later or earlier than the one they left. On arriving at their destination, their body might think it is 7:00 in the evening because that is the time it would be if they were at home. But because they have traveled to a new time zone, it might be midnight in the new location. So they are wide awake and not ready to be asleep for several more hours. Then when they need to wake up the next morning in this new time zone, they are short on sleep. People suffering jet lag may be groggy all day because their internal clock isn't lined up with the light and dark cues in their new environment. This can be an even greater problem if the actual time is seven, eight, or twelve hours behind or ahead of the body's clock. It can take days for the body's rhythms to catch up to the new location.

Each fall and each spring many states in the United States and several other countries change the clock time by an hour to make use of daylight hours. This is called setting the clocks for Daylight Saving Time (spring) or going back to Standard Time (fall). This too, can disrupt circadian rhythms for several days after the time change. On the Monday after a time change in the U.S., there is often an increase in heart attacks and car accidents, compared to other Mondays.

doctor visiting a patient on a bed

Night shift work also causes a disruption in the body clock and circadian rhythm. People with night jobs, such as doctors, nurses, security guards, firefighters and police officers, work during the night while the rest of us are asleep. It can be difficult for such workers to get their circadian clocks to match up with their outside world. At night, they must fight the body's natural rhythms as they try to stay awake. During the day, they have to try to sleep when their body expects to be alert. Their internal clock, which takes its cues form daytime light, often makes it hard for them to go to sleep and stay asleep during the day, even if they're tired. Night shift workers tend to be sleep-deprived.

person asleep at a computer

It is hard for people to catch up on sleep when they fall behind. For example, if someone misses two hours of sleep it is not really fixed by simply napping for two hours the next day. A good full night's sleep is the only way to solve the problem. Some people try to sleep longer on the weekends to make up for too little sleep during the week. But that also tends to throw off their circadian rhythms and make it even harder to wake up on time when Monday morning arrives.

Improving Our Sleep

Getting enough sleep is important to our health. A good night's sleep is just as important as healthy food and exercise. In fact, humans can survive longer without food than without sleep. Many health issues can be related to lack of sleep. But how do we improve our sleep? Getting into a routine for sleeping is one good way to make sure that we go to sleep, stay asleep and get the rest we need.

Parent Reading with Child

One thing that may help people sleep better is to avoid electronic devices in the hour just before bed. Don't text, watch TV or play video games right before bedtime. These activities can stimulate the brain in such a way that it has a hard time turning off to go to sleep. Also, remember that your internal clock can be disrupted by blue light from electronics. Studies have shown that reducing your exposure to screened devices can help you fall asleep more easily. A more relaxing routine of reading a book, taking a warm bath, enjoying quiet time or listening to soft music can help the body to shut down brain activity and allow sleep to come. Keeping to a consistent bedtime routine lets your body know that it is time for sleep.

Exercise has been shown to lessen the amount of time needed to fall asleep. This may be related to hormones produced by the body during exercise or even the fact that the body gets tired and ready for rest during a routine of exercise. Exercising just before bed, however, has often proved to make sleeping more difficult, so exercise earlier in the day.

Man Eating Spicy Food

Certain foods can interrupt sleep. Spicy food has been known to cause stomach discomfort just when the person is ready for sleeping. It may be best to avoid heavy meals before sleep and just have a glass of milk instead. Drinks containing caffeine such as cola, coffee, tea, and energy drinks may be too stimulating if consumed in the evenings.

Here are a few additional tips that may help to improve your sleep patterns:

No Noise Sign
  • Keep a constant schedule - go to bed and get up about the same time each day.

  • The room where you sleep should be quiet and dark. Darkness cues you to sleep.

  • Cooler room temperatures will help you stay asleep.

  • It may be best to keep your phone outside your bedroom at night.

  • Practices such as consciously relaxing each muscle of your body, visualizing calm thoughts, or “counting sheep” can help your brain wind down for sleep.

  • Try to get some sunlight each day - this helps regulate the body clock.

Dreams

Boy dreaming

We know that our most vivid dreaming happens during REM sleep. We can dream up to two hours every night. But what exactly are dreams, and why are they part of our sleep? Scientists are still trying to determine exactly why we dream and what purpose they have in our lives. Studies suggest that dreams actually help us to remember things that we have learned during the wakeful parts of our day. It may also be that dreams help us to deal with stress in our lives by helping us to practice how we might handle stressful situations. Your dreams may be related to how you feel, your worries, your hopes, things you are excited about or events that were important to you in the past. Movies and TV shows as well as real people and events can show up in your dreams, but usually in a jumbled way. Dreams often have randomness in the way they unfold, jumping from one event or location to another, or combining them in strange ways. This may be the brain's attempt at sorting and tying together events from our lives into one dream story. The events may have unusual aspects that represent unreal or fantasy-like parts. We don't usually notice this irrational dream story line until we are awake and are thinking about it or telling others about the dream.

Sometimes when we wake up from a dream, we say “But it seemed so real!” Belief in the unbelievable happens because in REM sleep, the brain is ruled by the deep-brain limbic system that involves emotions, not by the logic centers and impulse-control regions of the frontal lobe that are active when we are awake.

We may even be surprised to find people we have not had contact with for a long time in our dreams. Those people are often there because we had some type of encounter with a memory of them in our wakeful day, even if we were unaware. For example, while riding in the car you might see a restaurant where you had dinner with your cousin months ago. Although you are not really aware of this memory recall, the cousin appears in that night's dream.

child having a nightmare

Nightmares are a scary version of dreams and can be brought about by some type of stress in the person's life. These dreams can seem real to the sleeping person and often include smell, taste, pain, and loud sounds which rarely appear in standard dreams. Fear is a predominant emotion in nightmares.

People who are unable to dream or who do not sleep long enough to get to the dream stage have been found to have more problems with coordination, concentration, memory and their overall emotions and mood. Lack of dreaming, more than a lack of sleep, may cause severe depression. Although scientists don't fully understand why, dreams definitely serve a purpose for human health.

Sleep Disorders

Dreaming about Sheep

Many people have difficulties going to sleep and staying asleep. These problems can be just a temporary condition resulting from a short period of stress, too much stimulation just before bed, or even illness or pain. But long term sleep problems are called insomnia. Insomnia, the inability to fall asleep, can cause other health issues including weight gain, heart problems, diabetes, high blood pressure, and poor memory. Becoming ill due to a weakened immune system mean that colds, flu, and even pneumonia may also be linked to a lack of sleep.

Sleep Walking

Other sleep disorders include these unusual behaviors:

  • Sleep-eating disorders - While sleeping, the person will search for food in the house and eat, sometimes excessively.

  • Night terrors - Children sometimes experience this problem which includes screaming and fearful behavior while they are fully asleep. They usually have no memory of the event upon waking.

  • Sleepwalking - A person is asleep, but gets out of bed and tries to function as if awake. Some behavior during a sleep walking episode can seem odd or strange to observers.

  • Snoring - A sleeping person experiences a loud vibration of the soft tissue in the back of the mouth. It prevents the snorer from restful sleep and can have the same effect on others around them due to the disruptive sound of snoring.

  • Sleep Apnea - While sleeping, a person stops breathing for short periods because muscles in the throat obstruct or block the upper airway.

  • Narcolepsy - This is a brain-related problem in which the sleep/wake cycles are not well regulated. A person experiences random episodes of sleep during the day, and some people have “sleep attacks” and fall asleep very suddenly.

Sleep disorders should be discussed with a doctor who specializes in sleep problems. The sleep specialist may conduct a sleep study where the person's sleep is observed in order to treat the problem.

How Animals Sleep

All animals sleep, but sleep patterns vary widely throughout the animal kingdom. While the sleep habits of mammals and birds have been studied extensively, scientists know less about the sleep habits of reptiles, insects and fish. But it appears that all living organisms do have periods of rest when their activity level is reduced and their body functions slow down. In nature, it seems that no creature can sustain full activity 24 hours a day.

Sleeping dog

For mammals, time spent sleeping ranges from 2 hours a day (giraffes) to 20 hours a day (little brown bats.) In general, smaller mammals sleep more than larger animals. Nearly all mammals spend time in non-REM sleep and in REM sleep, suggesting that other mammals dream, just as humans do. Some animals (like humans) get their sleep in one long time period each day, while others sleep in multiple short periods throughout a 24-hour cycle. Dogs tend to nap in 80-minute segments throughout the day, while gorillas can sleep for 12 hours in darkness. Elephants can go for days without sleep when traveling from one place to another. Learn more about animals' sleep needs with this comparison chart.

Animals have different kinds of internal clocks and sleep at different times of the day. Some, like humans and most birds, are diurnal, sleeping mostly at night and staying active throughout the day. Others, like raccoons and bats, are nocturnal, sleeping during the day and becoming active at night. Still others, like deer, are crepuscular, being most active at sunset or just before dawn. A few animals, such as lemurs, switch from diurnal to nocturnal depending on the season.

Dolphin

Many animals have developed amazing adaptations that allow them to sleep while also remaining alert to danger and maintaining bodily functions. Dolphins, for example, sleep with one eye open. When dolphins sleep, half their brain is awake while the other half sleeps, allowing them to surface for air and watch out for predators while continually swimming. Frigate birds, who fly over the ocean for months while migrating, can nap while gliding. Nurse sharks rest in a pile on the ocean floor. Horses sleep part of the time standing up (during NREM sleep) and part lying down (during REM sleep.) Fish and snakes sleep with their eyes open. Otters often hold hands while they sleep so they don't drift away from each other. Gorillas and orangutans build sleeping platforms high in the trees, where they are can sleep safely throughout the night. Walruses sleep in a vertical position in the water, often digging their tusks into ice in order to stay in place while they sleep. Birds are able to perch on tree branches while they sleep because tendons in their feet lock, preventing them from falling. Mallard ducks sleep in a row, with the ducks on the ends of the row keeping watch while the others sleep.

Bear hibernating

Hibernation (and variations of it) is a type of sleep that many animals participate in during the winter months. Their body temperature drops significantly, their breathing slows way down, and they may be very difficult to wake. Body systems shut down in many hibernating animals so that they don't need to eat, poop, or pee. Others, especially small animals, will wake for brief periods of time to snack on food they have stored, then go back to sleep. Why do they hibernate? For many it is difficult to find food in winter, and hibernation allows them to conserve body energy so they don't need as much food. Bears, frogs, mice, ground squirrels, skunks, bats, and many reptiles sleep during the winter in some form of hibernation behavior. A few animals also survive extended periods of heat or drought with a form of hibernation known as estivation. For example, the West African lungfish digs a burrow into the mud where it creates a mucous sack to live in that allows it to survive the dry season. When water returns to the area, the lungfish sheds the sack and returns to normal. Other estivating animals include some species of frogs, salamanders and crocodiles.

Humans are the only animals that avoid or delay sleep on purpose. Maybe we should take a lesson from the rest of the animal world and recognize that sleeping according to our internal clocks is an essential part of a healthy and productive life.

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