Sleep: What It Is and Why It’s Important

Despite spending nearly one third of our lives sleeping, the exact reason why we do it is still a mystery.1,2 There is ample research surrounding sleep: why we need it, how much we need, and the effects of its deprivation. But when it comes to pinning down exactly what sleep is and why we need it, the answers are murky at best.


As recently as the 1950s, most people thought of sleep as a passive, dormant state, but researchers have found that our brains are actually very active during sleep.4 How is this possible? It comes down to neurotransmitters. These nerve- signaling chemicals control whether we are asleep or awake by acting on different groups of nerve cells, called neurons, in the brain.4 Neurons produce neurotransmitters, such as serotonin and norepinephrine, that keep parts of the brain active while we are awake. Other neurotransmitters produce hormones, such as melatonin, that signal to fall asleep. You can think of these mechanisms as two levers – one for wakefulness and one for sleepiness. Each lever goes up or down in a continuum as one affects the other.5 In other words, there is no binary switch for being asleep and being awake. These neurotransmitters undulate throughout the day, from the time you wake up in the morning until you fall asleep at night.


While there is still much to be learned about the intricacies of how sleep works, existing research has shed some light on the dynamic mechanics of what happens to the brain and body during hours of slumber.1 While you sleep, you progress through multiple sleep cycles, each of which lasts between 70 and 120 minutes.6 These cycles each consist of separate stages, which are fundamental to how sleep works.

There are two basic types of sleep: rapid eye movement (REM) and non-REM, each of which is distinguished by its own brain wave pattern.1,6 Non-REM sleep has three cycles, and normally you will cycle through all stages of non-REM and REM sleep throughout night, with increasingly longer, deeper REM periods occurring toward morning.2

Stage 1.

This is the transition from wakefulness to light sleep. Your body begins to slow down and loosen up. During these few minutes, your heartbeat, breathing, and eye movements begin to slow, and your muscles relax with occasional twitches.2,6 Your brain waves begin to slow from their daytime wakefulness patterns.1

Stage 2.

N2 is an extension of the first stage. It is a period of light sleep before you enter deeper sleep. Your heartbeat and breathing slow and muscles relax further. Your body temperature drops, and your eye movements stop.2,3 Brain wave activity slows but has brief bursts of electrical activity.1 This is the most repeated sleep cycle.

Stage 3.

N3 is the deepest part of the first three stages and occurs for longer periods of time during the first half of the night.3 Your body spends about 25 percent of its time in deep sleep. This stage is what you need to feel refreshed and chipper in the morning.1,2 Your heartbeat and breathing slow to their lowest levels, and your muscles are relaxed. Brain waves become even slower.

Stage 4.

REM sleep, also known as dream sleep, first occurs during the first 90 minutes after falling asleep. It may seem like your body is being tricked awake. Your eyes move rapidly behind closed eyelids and brain wave activity becomes closer to that seen in wakefulness.5,6 Your breathing becomes faster and irregular, and your heart rate and blood pressure increase to near waking levels.2 This is when most of your dreaming occurs.3 Your arm and leg muscles become temporarily paralyzed, which prevents you from acting out your dreams. This stage reoccurs about every 60 to 90 minutes, lasting around 25 to 45 minutes, increasing in length as the night goes on.3


While experts haven’t reached a consensus on why we sleep, there is plenty of evidence supporting the concept that it is an essential biological function. 5 Sleep allows your body to recharge and repair itself and prepares it to take on another day. 8,9 The importance of sleep is most evident when you understand what happens to the body when there is a chronic lack of it or sleep deprivation.

Cardiovascular health. Lack of sleep impacts your body’s ability to heal and repair blood vessels, which play important roles in controlling blood sugar and blood pressure.10 Lack of sleep has also been associated with higher levels of inflammation, which can trigger a number of cardiovascular stressors including, stroke, heart attack, hypertension, and Type 2 diabetes.11,12

Weight. Sleep helps regulate the levels of two hormones, leptin and ghrelin, which control feelings of hunger and fullness. Leptin tells your brain that you’ve had enough to eat. However, without enough sleep, your brain reduces leptin and raises ghrelin, which is an appetite stimulant.10 Furthermore, lack of quality sleep can cause fatigue, which can negatively affect your motivation to exercise. These compounding factors can lead to unhealthy weight gain.

Brain function. Sleep affects your central nervous system (CNS), which is the information highway of your body, and sleep deprivation can seriously disrupt how your body sends and processes information.10 The pathways between neurons that form during sleep keep your brain sharp, so a lack of rest can make it more difficult to concentrate and absorb information. It can also compromise decision-making processes and creativity.10 A lack of sleep has effects on your gross motor functioning as well. Signals from your brain may be delayed, resulting in decreased coordination and balance. Sleep deprivation can even cause microsleeps, very brief episodes of sleep (i.e., nodding off ) during normal awake time, which can have deadly consequences for you and others (e.g., if they occur while driving or operating heavy machinery).10,13

Psychological health. Sleep deprivation negatively affects your mental and emotional state. Even after one night of disrupted sleep, you may experience daytime drowsiness, irritability, mood swings, and anxiety. Several days of disrupted sleep can result in impaired judgment and trigger impulsive behavior, depression, confusion, and paranoia.10 In fact, 48 hours or more without sleep has been associated with perceptual distortions and hallucinations, even in those with no history of mental illness.14 Because mind, mood, and physical health are so closely intertwined, it should come as no surprise that sleep deprivation has a cascading domino effect on the rest of the body.

Immune system. Sleep cannot necessarily prevent you from getting sick, but a lack of it can weaken your immune system.15 It’s a two-way street—immune factors help regulate sleep and vice versa.16 Insufficient sleep can result in reduced production of cytokines, a type of protein that fights infection and regulates inflammation. Because cytokines are both produced and released during sleep, this can cause a double whammy of immune weakness when you’re already feeling exhausted.15

NREM Stage 1 N1 1–5 minutes
NREM Stage 2 N2 10–60 minutes
NREM Stage 3 N3, Slow-Wave Sleep (SWS), Delta Sleep, Deep Sleep 20–40 minutes
REM Stage 4 REM Sleep, Dream Sleep 10–60 minutes

Category Age Group Recommendation (Hours)
Newborns 0-3 months 14–17
Infants 4-11 months 12–15
Toddlers 1-2 years 11–14
Preschoolers 3-5 years 10–13
School age children 6-13 years 9–11
Teenagers 14-17 years 8–10
Younger adults 18-25 years 7–9
Adults 26-64 years 7–9
Older adults 65+ years 7–8


Numerous health organizations have made suggestions on the amount of sleep a human needs. In 2015, the National Sleep Foundation conducted a scientifically rigorous update to their sleep duration recommendations, which serve as a reputable, comprehensive standard.17 See the table for NSF’s recommendations.

These sleep recommendations represent guidelines for healthy individuals and those not suffering from a sleep disorder.17 However, sleep needs vary person to person. The NSF recommendations are not a “one size fits all” scenario. The commonly accepted eight-hour sleep average is

just that—an average. Sleep needs can vary based on quality of sleep, physical activity, circumstances, and certain health conditions.5 It is worth noting, however, that a drastic deviation from the normal range is unlikely in healthy person.17


While it well established that quality sleep is necessary for maintaining physical and mental health, the majority of Americans find themselves deprived of it.18 In fact, the American Sleep Foundation surveyed a random national sample of 1,011 adults and published their results in their Sleep in America® poll.18-20 Here are some of their key findings:20

  • Americans feel sleepy at some point during normal waking hours an average of three days a week, with many individuals reporting that daytime sleepiness negatively impacts their daily activities, mood, mental acuity, and productivity.
  • Just 16 percent of those surveyed reported not feeling sleepy at all during normal waking hours in a typical week, excluding bedtime and waking up in the morning.
  • Biological sex may play a role sleep. Men and women report feeling sleepy 2.7 and 3.4 days a week on average, respectively.
  • The most common effects of sleepiness reported were fatigue, mood instability, headaches, and feeling generally unwell.
  • In terms of coping with sleepiness, 62 percent of those sampled tried to simply to “shake off ” sleepiness. Other common ways people reported counteracting daytime sleepiness include getting fresh air, drinking coffee or a caffeinated beverage, and taking a nap.


Despite the innumerable causes of sleep disruption and deprivation, there is expert consensus, namely among the CDC, the National Institutes of Health, the National Institute on Aging, and the American Academy of Family Physician, that point to strategies that promote more restful sleep.18 Here are some of the fundamental tips for better sleep:

  1. Design your environment: Your senses, especially sight, sound, and touch, can determine the quality of sleep you get each night. For example, excess light exposure can throw off your sleep and circadian rhythm.18 You can counteract this by investing in blackout curtains or a sleep mask to block light and prevent it from interfering with your rest.18 Make sure to keep noise to a minimum.3,21 If you can’t avoid it, a fan or white noise machine can drown out distracting sounds. Earplugs or headphones are another adequate option. Finally, keep your bedroom a comfortable temperature. Some research has shown that the ideal room temperature is cooler, resting around 65o Fahrenheit.3,18
  2. Stick to a set wake-up time. It’s nearly impossible for your body to have a healthy, sustainable sleep schedule if you’re constantly waking up at different times.21 This is why it is important to pick a consistent wake-up time and stick with it. This also applies to weekends or other days when you aren’t necessarily required to get up at a specific time. If you do need to change your sleep schedule, do it gradually by adjusting it by 1 or 2 hours a night. This will make the change more sustainable.18
  3. Set the mood. It’s much easier to fall asleep if your mind and body gradually unwind. You can start this process about 30 minutes prior to getting into bed. Avoid consuming caffeinated foods or drinks later in the day because they can reduce the quantity and quality of sleep. According to the American Academy of Sleep Medicine, you should avoid caffeine at least six hours before bedtime.22 Similarly, keep in mind that alcohol is a sedative, not a sleeping agent.5 While it can make you drowsy, it lowers sleep quality by inducing light sleep instead of the deep sleep you need to wake up refreshed the next morning. Turning down bright light also helps you transition to bedtime and contributes to melatonin production.18 Finally, avoid blue light, which suppresses your natural production of melatonin, by turning off your devices, including phones, computers, and TVs, at least 30 minutes before heading to bed.
  4. Create time for sleep. Budget your time to get the recommended amount of sleep each night by incorporating sleep hours into your schedule just like you would with any other important task. Do this by working backwards—look at your fixed wakeup time and subtract the hours of sleep you need to identify a target bedtime.18 How do naps fit into this equation? That’s about timing.
  5. Do not nap for too long or too late in the day. This can throw off your sleep schedule and make it harder to get to sleep at bedtime.18 The best time to nap is shortly after lunch in the early afternoon for no more than 20 to 30 minutes. 3,5,18,23
  6. Don’t stew in bed if sleep doesn’t happen. Keep your bedroom a place for sleeping and intimacy.18,23 Try to avoid a connection in your mind between your bedroom and frustration from sleeplessness. If you’ve spent more than 20 minutes trying to fall asleep, get out of bed and do something relaxing in low light.18 Try reading or listening to a book, meditating, or practicing controlled breathing exercises. Avoid sneaking glances at your clock by switching off its light or turning it to face the wall. One of the main causes of insomnia is anxiety and fear of … not being able to fall asleep.3 Avoiding this cycle is easier said than done. If you feel like stress is chronically preventing you from quality sleep, be proactive. Schedule an appointment with your doctor and mention stress during your next checkup.23
  7. Find time to move. It’s a lot easier to fall asleep when you’re exhausted from the day’s activities. In addition to its many other health benefits, daily exercise promotes quality sleep., especially if you can exercise outdoors. 18 Because our internal clock is regulated by our light-dark exposure, of which daylight has the strongest effect, 11. exercising outdoors during daylight hours, especially if done early in the day, can help normalize circadian rhythm. Try to avoid intense  exercise close to bedtime, however, because the immediate after- effects of exercise tend to promote wakefulness, not sleepiness.


REM sleep was discovered by Eugene Aserinsky, a graduate student in physiology at the University of Chicago, in the 1950s.7 Aserinsky brought his eight-year-old son, Armond, to work one day for a sleep study. Using an ancient brain-wave machine called an Offner dynograph, Aserinsky taped electrodes to his son’s head and plugged the leads into a switch box over the bed.8 In an adjacent room, Aserinsky calibrated the machine, telling his son to look left, right, up, and down, and the ink pens moved across the paper in unison with the boy’s eyes. A few hours after the boy fell asleep, the pens tracking his son’s eye movements and brain activity began moving.8 Thinking his son had woken up, Aserinsky checked in on him and was surprised to find his son was still fast asleep. Aserinsky published his findings in 1953, which resulted in scientists rethinking the concept of sleep. Prior to Aserinsky’s the discovery, sleep thought to be an inactive, passive state.2 This new concept of REM sleep led researchers to discover the distinct electrical and biochemical stages of the sleeping brain,7 which eventually led to new branch of medicine specifically focused on sleep that has since helped doctors better diagnose and treat sleep disorders.

Microsleeps and Drowsy Driving

Microsleeps, which are caused by extreme fatigue, are especially dangerous when it comes to drowsy driving. Symptoms include nodding off, reacting more slowly to changing road conditions and slowed decision making, experiencing tunnel vision, drifting from your lane, and forgetting the last few miles you have driven.

In fact, drowsy driving is not a far cry from drunk driving. After 17 consecutive hours awake, impairment from driving fatigue is equivalent to having a blood alcohol content (BAC) of 0.05. After24 consecutive hours awake, this impairment is equivalent to a BAC of 0.10.25 Drowsy driving is responsible as many as 20% of fatal car accidents in the U.S.


  1. Suni E. How sleep works. The National Sleep Foundation website. Updated October 23, 2020. Accessed October 30, 2020
  2. Office of Communications and Public Liaison. Brain basics: understanding sleep. National Institute of Neurological Disorders and Stroke website. Accessed November 5, 2020.
  3. Ward A, Winter WC. Somnology (SLEEP): part I with Dr. W. Chris Winter Ologies podcast. Ologies website. Updated October 15, 2018. Accessed October 26, 2020.
  4. What is sleep and why is it important? The American Sleep Association website. Accessed November 5, 2020.
  5. Ward A, Winter WC. Somnology (SLEEP): part II with Dr. W. Chris Winter Ologies podcast. Ologies website. Updated October 22, 2018. Accessed October 26, 2020.
  6. Patel AK, Reddy V, Araujo JF. Physiology: sleep stages. National Center for Biotechnology Information and U.S. National Library of Medicine website. Updated April 29, 2020. Accessed November 8, 2020. 
  7. Peever J, Fuller PM. Neuroscience: a distributed neural network controls REM sleep. Curr Biol. 2016;26(1):R34–R35. 
  8. Brown C. The stubborn scientist who unraveled a mystery of the night. Smithsonian Magazine website. Updated October 2003. Accessed November 10, 2020.
  9. Suni E. Why do we need sleep? The National Sleep Foundation website. Updated September 11, 2020. Accessed November 10, 2020.
  10. Watson S, Cherney K. The effects of sleep deprivation on your body. Healthline website. Updated May 15, 2020. Accessed November 10, 2020.
  11. Harvard Medical School. Harvard Men’s Health Watch website. Playing with the fire of inflammation. Updated October 10, 2019. Accessed November 10, 2020.
  12. Center for Disease Control and Prevention website. How does sleep affect your heart health? Updated December 3, 2018. Accessed November 10, 2020.
  13. Poudel GR, Innes CR, Bones PJ, et al. Losing the struggle to stay awake: divergent thalamic and cortical activity during microsleeps. Hum Brain Mapp. 2014;35(1):257–269. 
  14. Waters F, Chiu V, Atkinson A, Blom JD. Severe sleep deprivation causes hallucinations and a gradual progression toward psychosis with increasing time awake. Front Psychiatry. 2018;9:303. 
  15. The National Sleep Foundation website. How sleep affects your immunity. Accessed November 10, 2020
  16. Rogers NL, Szuba MP, Staab JP, et al. Neuroimmunologic aspects of sleep and sleep loss. Semin Clin Neuropsychiatry. 2001;6(4):295–307.
  17. Hirshkowitz M, Whiton K, Albert SM, et al. National Sleep Foundation’s sleep time duration recommendations: methodology and results summary. Sleep Health. 2015;1(1):40–43.
  18. Suni E. Healthy sleep tips. Updated July 30, 2020. The National Sleep Foundation website. Accessed November 10, 2020.
  19. The National Sleep Foundation website. The National Sleep Foundation’s 2020 Sleep in America® poll shows alarming level of sleepiness and low levels of action. Updated July 30, 2020. Accessed November 10, 2020.
  20. The National Sleep Foundation website. Americans feel sleepy 3 days a week, with impacts on activities, mood & acuity. Accessed November 10, 2020.
  21. American Academy of Sleep Medicine website. Healthy sleep habits. Updated February 9, 2017. Accessed November 10, 2020.
  22. Heffron TM. Sleep and caffeine. Updated August 1, 2013. American Academy of Sleep Medicine website.,be%20stronger%20in%20older%20adults. Accessed November 11, 2020.
  23. Suni E. Stress and insomnia. Updated September 17, 2020. The National Sleep Foundation website. Accessed November 11, 2020.
  24. Driver Fatigue. The Center for Disease Control and Prevention website. Updated February 27, 2020. Accessed November 12, 2020.
  25. Driver Fatigue on the Job. The National Institute for Occupational Safety and Health (NIOSH) website. Updated September 22, 2020. Accessed November 12, 2020.
  26. Suni E. Dreams. The National Sleep Foundation website. Updated October 30, 2020. Accessed November 12, 2020.

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