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What Is Sleep

What Is Sleep?

What Is Sleep?

Sleep has distinctive stages that cycle throughout the night in predictable patterns (4). Each stage of sleep is associated with specific types of brain waves (patterns of electrical activity in the brain). Sleep is divided into two basic types: rapid eye movement sleep (REM) sleep and non-REM sleep (with three distinctive stages) (4).

Sleep usually begins with non-REM sleep. In stage 1 of non-REM sleep, your brain waves, heart, and breathing slow, your eyes move slowly, and your muscles relax (4). You sleep lightly and can be easily awakened by noise or other disturbances.

Then you enter stage 2 of non-REM sleep where your brain waves are even slower but with occasional bursts of rapid waves (4). Your eye movements stop. You remain in this stage for about half the night.

Stage 3 of non-REM sleep occurs soon after you fall asleep and mostly in the first half of the night (4-5). During this stage, your brain waves slow even more and the brain almost exclusively produces extremely slow waves (called Delta waves). This is a very deep stage of sleep and it is difficult to be awakened.

REM sleep first occurs about 60-90 minutes after you fall asleep and after that it cycles along with the non-REM stages of sleep throughout the night, with longer, deeper periods of REM occurring during the second half of the night (5-6). As you sleep, REM sleep time becomes longer while stage 3 non-REM sleep becomes shorter; therefore nearly all of your sleep time has been spent in stages 1 and 2 of non-REM and in REM sleep (6). Overall, almost half of your total sleep time is spent in stage 2 non-REM sleep and one-fifth in stage 3 non-REM sleep (6). During REM sleep, your eyes move rapidly though they remain closed, your breathing becomes rapid, shallow, and irregular, and your heart rate and blood pressure increase. Dreaming typically occurs during REM sleep and your arms and legs are temporarily paralyzed so that you cannot “act out” any dreams.

Why REM sleep is so important and why people dream is unclear, but it is known that REM sleep stimulates the brain regions that allow you to learn and make memories, though other stages of sleep besides REM are also necessary to forming pathways in the brain that enable people to learn and remember (6).

What Makes You Sleep

If you are not getting it, the need for sleep soon becomes overwhelming. You cannot adapt to getting less sleep because your body’s internal processes require a certain amount of sleep (NIH, 7). Scientists believe that two substances your body produces are, at least in part, responsible for this need for sleep.

One, called adenosine, builds up in your blood while you are awake and is broken down by your body while you sleep. Levels of this substance in your body may help trigger sleep when needed. A buildup of adenosine and other complex factors might explain why, after several nights of insufficient sleep, you build a sleep debt, which may cause you to sleep longer than normal or at unplanned times throughout the day.

The second substance is melatonin. This hormone is part of your “biological clock,” a small bundle of cells in your brain that control your sleep patterns. Internal and external cues, such as light signals received through your eyes, control these cells. Your biological clock triggers your body to produce melatonin, which helps prepare your body for sleep. Your biological clock makes you feel the most tired between midnight and 7 a.m. You may also feel sleepy between 1 p.m. and 4 p.m. when another increase in melatonin occurs in your body.

What Sleep Does For You

Sleep affects your learning, memory, and mood (NIH, 12). Sleep is necessary for creating and maintaining learning and memory pathways in the brain. Recent studies show that people can learn a task better and better remember what they learned if they are well-rested. In one study, volunteers had to sleep at least 6 hours to show improvement in learning, and the amount of improvement was directly related to how much time they slept; volunteers who slept 8 hours outperformed those who slept only 6 or 7 hours (12). A lack of sleep causes thinking to slow down and makes it difficult to focus and pay attention and can lead to faulty decision-making and more risk-taking (13). It also slows your reaction time, which is particularly important when driving and other tasks that require quick response. When sleep deprived people are tested on a driving simulator, they perform just as poorly as people who are drunk (13)

Also, most people with a sleep debt report being irritable and even unhappy and those who chronically suffer from a lack of sleep are at a greater risk for developing depression (13). In fact, some experts think that post-partum depression is caused, in part, by a lack of sleep.

Sleep gives your cardiovascular system, including your heart, a needed rest. During non-REM sleep your heart rate and blood pressure slow more and more as you move into deeper sleep (13). During REM sleep your heart rate and breathing rates can rise and fall with your dreaming (13-14). These changes seem to promote cardiovascular health (14).

A lack of sleep also puts your body under stress and can trigger the release of more adrenaline, cortisol, and other stress hormones during the day, which can keep your blood pressure from dipping during sleep, increasing your risk for heart disease (14). Lack of sleep can also cause your body to produce more of certain proteins that are thought to play a role in heart disease (14).

Sleep helps keep your body strong and healthy. Deep sleep (stage 3 non-REM sleep) triggers more release of growth hormone, which contributes to growth in children and boosts muscle mass and the repair of cells and tissues in children and adults (14). Also, during sleep your body produces more cytokines—cellular hormones that help the immune system fight various infections; lack of sleep can reduce your body’s ability to fight infections (14). For example, sleep-deprived volunteers given the flu vaccine produced less than half as many flu antibodies as those given the same vaccine who were well-rested (15).

Studies also suggest that sleep is a regulator of appetite, energy use, and weight control (15). During sleep, the body increases its production of the appetite suppressor leptin and decreases its production of the appetite stimulant grehlin (15). The less people sleep, the more likely they are to be overweight or obese and prefer eating foods that are higher in calories and carbohydrates (15). For example, people who sleep 5 hours a night on average are much more likely to become obese than people who sleep 7-8 hours a night (15).

Several hormones released during sleep control your body’s use of energy. A distinct rise and fall of blood sugar levels during sleep appears to be linked to sleep stages (15). Not sleeping at the right time or getting enough overall sleep or enough sleep in each stage disrupts this pattern. For example, one study found that when healthy men slept only 4 hours a night for 6 nights in a row, their insulin and blood sugar levels matched those seen in people who were developing diabetes (15).

What happens when you don’t get enough sleep

Research suggests adults need at least 7-8 hours of sleep each night to be well rested (2). Unfortunately, in our modern, get-it-now culture, sleep is often the first thing people cut down on to make room for other activities. Over the last several decades, sleep research studies have shown that sleep is not just a period during which our brains shut down. Many vital activities that keep you healthy take place during sleep. For example, your brain forms the pathways necessary for learning and creating memories and new insights during sleep. A lack of sleep may impair your ability to focus, pay attention, and respond quickly and may cause mood problems (NIH, 1). More than one-third of adults report daytime sleepiness so severe that it interferes with work, driving, and social functioning at least a few days each month (2).Recent studies suggest that a chronic lack of sleep can even increase your risk of obesity, diabetes, cardiovascular disease, and infections (1). Chronic sleep loss or disorders may affect as many as 70 million Americans, resulting in an annual cost of $16 billion in health care expenses and $50 billion in lost productivity (2).

How Much Sleep Is Enough

Sleep needs change throughout the life cycle. Newborns sleep between 16 and 18 hours a day, children in pre-school sleep between 11 and 12 hours a day, and school-aged children and adolescents need at least 10 hours of sleep each night (19). When healthy adults are given unlimited opportunity to sleep, they sleep on average between 8 and 8.5 hours a night, but sleep needs vary person to person (19). Some people need only about 7 hours while others need 9 or more hours of sleep a night, but most adults need 7-8 hours a night (19).

(See “Your Guide to Healthy Sleep” U.S. Department of Health and Human Services, National Heart, Lung, and Blood Institute.)
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