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11.2: What is Sleep?

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    Learning Objectives
    1. Evaluate the overall complex nature of sleep, rather than simply the absence of wakefulness.
    2. Demonstrate a general understanding of the biological nature of sleep.
    3. Evaluate the importance of sleep, its functions and the negative ramifications of sleep deprivation.


    When you are asleep are you unconscious? Why do all animals appear to need sleep, some at great expense and danger? Is it true that we "need" sleep every night, or are we just wasting time (about a third of our lives if we follow the 8 hours a day recommendation)? Really all we are doing is laying down and turning off the lights, right? And our brain just shuts off for that time, right?

    The nature of sleep as a biological process that is generated by the brain will be discussed in this chapter. In this section, some ideas about how these discoveries came about are contrasted against people's general belief that sleep is just something that happens when one is not doing anything else.

    What is Sleep?

    Sleep is a complex biological process. While you are sleeping, you may appear unconscious (though there is certainly a difference between being asleep and having fainted), but your brain and body functions are still active. They are doing a number of important jobs that help you stay healthy and function at your best. So when you don't get enough quality sleep, it does more than just make you feel tired. It can affect your physical and mental health, thinking, and daily functioning.

    Sleep is an important part of your daily routine—you spend about one-third of your time doing it. Quality sleep – and getting enough of it at the right times -- is as essential to survival as food and water.

    Everyone needs sleep, but its full biological purpose remains a mystery. Sleep affects almost every type of tissue and system in the body – from the brain, heart, and lungs to metabolism, immune function, mood, and disease resistance. Research shows that a chronic lack of sleep, or getting poor quality sleep, increases the risk of disorders including high blood pressure, cardiovascular disease, diabetes, depression, and obesity.

    Sleep is important to a number of brain functions, including how nerve cells (neurons) communicate with each other. In fact, your brain and body stay remarkably active while you sleep. Recent findings suggest that sleep plays a housekeeping role that removes toxins in your brain that build up while you are awake (Xie et al, 2013).

    Without sleep you can’t form or maintain the pathways in your brain that let you learn and create new memories, and it’s harder to concentrate and respond quickly.

    Sleep is a complex and dynamic process that affects how you function in ways scientists are now beginning to understand.

    Functions of sleep

    Memory consolidation

    Another idea about why we sleep is that sleeping reinforces learning and memory, while at the same time, helping us to forget or to clear stores of unneeded memories. During the course of a day we are inundated with experiences, some of which should be remembered while others need not be. Perhaps sleep aids in rearranging all of the experiences and thoughts from the day so that those that are important are stored and those that are not are discarded. People who get plenty of deep NREM sleep in the first half of the night and REM sleep in the second half improve their ability to perform spatial tasks. This suggests that the full night's sleep plays a role in learning—not just one kind of sleep or the other.

    Other research has also shown that activity in the hippocampus during REM sleep supports the idea that dreams serve a memory consolidation function. Further the activity of the inferior parietal lobule, a part of the cortex that conveys experiences to memory, decreases during REM sleep, which probably helps to explain why we have so much trouble in remembering our dreams. There is also a lot of activity noted in the limbic regions – particularly the amygdala, probably in relation to the emotional quality of dreams that we report.

    Research with rats, cats, songbirds, and humans have shown that multiple stages in sleep increase memory consolidation. Rasch and Born (2013) reviewed the multiple genetic, neurological, and chemical bases by which memory function is affected during sleep.

    Studies show that a good night's sleep improves learning. Whether you're learning math, how to play the piano, how to perfect your golf swing, or how to drive a car, sleep helps enhance your learning and problem-solving skills. Sleep also helps you pay attention, make decisions, and be creative.

    Energy Conservation

    This theory states that we sleep to conserve energy and is based on the fact that the metabolic rate is lower during sleep. The theory predicts that total sleep time and NREM sleep time will be proportional to the amount of energy expended during wakefulness. Support for this theory is derived from several lines of evidence. For example, NREM and REM sleep states are found only in endothermic animals (that is, those that expend energy to maintain body temperature). Species with greater total sleep times generally have higher core body temperatures and higher metabolic rates. Consider also that NREM sleep time and total sleep time decrease in humans, with age, as do body and brain metabolism. In addition, infectious diseases tend to make us feel sleepy. This may be because molecules called cytokines, which regulate the function of the immune system, are powerful sleep inducers. It may be that sleep allows the body to conserve energy and other resources, which the immune system may then use to fight the infection.

    Brain Development

    This proposed function of sleep is related to REM sleep, which occurs for prolonged periods during fetal and infant development. This sleep state may be involved in the formation of brain synapses.

    Children and teens who are sleep deficient may have problems getting along with others. They may feel angry and impulsive, have mood swings, feel sad or depressed, or lack motivation. They also may have problems paying attention, and they may get lower grades and feel stressed.

    Discharge of emotions

    Perhaps dreaming during REM sleep provides a safe discharge of emotions. As protection to ourselves and to a bed partner, the muscular paralysis that occurs during REM sleep does not allow us to act out what we are dreaming. Additionally, activity in brain regions that control emotions, decision making, and social interactions is reduced during sleep. Perhaps this provides relief from the stresses that occur during wakefulness and helps maintain optimal performance when awake.

    Sleep Deprivation and Deficiency

    Why Is Sleep Important?

    Whichever theory is most effective, it is clear that sleep plays a vital role in good health and well-being throughout your life. Getting enough quality sleep at the right times can help protect your mental health, physical health, quality of life, and safety.

    The way you feel while you're awake depends in part on what happens while you're sleeping. During sleep, your body is working to support healthy brain function and maintain your physical health. In children and teens, sleep also helps support growth and development.

    The damage from sleep deficiency can occur in an instant (such as a car crash), or it can harm you over time. For example, ongoing sleep deficiency can raise your risk for some chronic health problems. It also can affect how well you think, react, work, learn, and get along with others.

    Healthy Brain Function and Emotional Well-Being

    As seen above, sleep helps your brain work properly. While you're sleeping, your brain is preparing for the next day. It's forming new pathways to help you learn and remember information.

    Studies also show that sleep deficiency alters activity in some parts of the brain. If you're sleep deficient, you may have trouble making decisions, solving problems, controlling your emotions and behavior, and coping with change. Sleep deficiency also has been linked to depression, suicide, and risk-taking behavior.

    Physical Health

    Sleep plays an important role in your physical health. For example, sleep is involved in healing and repair of your heart and blood vessels. Ongoing sleep deficiency is linked to an increased risk of heart disease, kidney disease, high blood pressure, diabetes, and stroke.

    • Sleep deficiency also increases the risk of obesity. For example, one study of teenagers showed that with each hour of sleep lost, the odds of becoming obese went up. Sleep deficiency increases the risk of obesity in other age groups as well.
    • Sleep helps maintain a healthy balance of the hormones that make you feel hungry (ghrelin) or full (leptin). When you don't get enough sleep, your level of ghrelin goes up and your level of leptin goes down. This makes you feel hungrier than when you're well-rested.
    • Sleep also affects how your body reacts to insulin, the hormone that controls your blood glucose (sugar) level. Sleep deficiency results in a higher than normal blood sugar level, which may increase your risk for diabetes.
    • Sleep also supports healthy growth and development. Deep sleep triggers the body to release the hormone that promotes normal growth in children and teens. This hormone also boosts muscle mass and helps repair cells and tissues in children, teens, and adults. Sleep also plays a role in puberty and fertility.
    • Your immune system relies on sleep to stay healthy. This system defends your body against foreign or harmful substances. Ongoing sleep deficiency can change the way in which your immune system responds. For example, if you're sleep deficient, you may have trouble fighting common infections.

    Daytime Performance and Safety

    Getting enough quality sleep at the right times helps you function well throughout the day. People who are sleep deficient are less productive at work and school. They take longer to finish tasks, have a slower reaction time, and make more mistakes. After several nights of losing sleep—even a loss of just 1–2 hours per night—your ability to function suffers as if you haven't slept at all for a day or two.

    Lack of sleep also may lead to microsleep. Microsleep refers to brief moments of sleep that occur when you're normally awake. You can't control microsleep, and you might not be aware of it. For example, have you ever driven somewhere and then not remembered part of the trip? If so, you may have experienced microsleep. Even if you're not driving, microsleep can affect how you function. If you're listening to a lecture, for example, you might miss some of the information or feel like you don't understand the point. In reality, though, you may have slept through part of the lecture and not been aware of it.

    Who Is at Risk for Sleep Deprivation and Deficiency?

    Sleep deficiency, which includes sleep deprivation, affects people of all ages, races, and ethnicities.

    Sleep deficiency can cause problems with learning, focusing, and reacting. You may have trouble making decisions, solving problems, remembering things, controlling your emotions and behavior, and coping with change. You may take longer to finish tasks, have a slower reaction time, and make more mistakes.

    The signs and symptoms of sleep deficiency may differ between children and adults. Children who are sleep deficient might be overly active and have problems paying attention. They also might misbehave, and their school performance can suffer.

    Sleep-deficient children may feel angry and impulsive, have mood swings, feel sad or depressed, or lack motivation.

    You may not notice how sleep deficiency affects your daily routine. A common myth is that people can learn to get by on little sleep with no negative effects. However, research shows that getting enough quality sleep at the right times is vital for mental health, physical health, quality of life, and safety.


    It is clear that sleep is a complex biological process that all humans, indeed all animals, engage in. Several brain regions are involved in sleep and wakefulness processes, and sleep serves complex functions that continue to be researched. The seemingly simple idea that we put on our pajamas, close our eyes and nod off to sleep involves far more complexity than appears on the surface.


    Rasch, B. & Born, J. (2013). About sleep's role in memory. American Psychological Society: Physiological Reviews, 93(2).

    Xie, L., Kang, H., Xu, Q., Chen, M.J., Liao, Y. Thiyagarajan, M., O'Donnell, J., Christensen, D.J., Nicholson, C., Iliff, J.J., Takano, T., Deane, R., Nedergaard, M. (2013). Sleep drives metabolite clearance from the adult brain. Science, 342 (6156), 373-377. doi:10.1126/science.1241224


    This page titled 11.2: What is Sleep? is shared under a mixed license and was authored, remixed, and/or curated by ASCCC OERI & Bakhtawar Bhadha (ASCCC Open Educational Resources Initiative (OERI)) .