Sleep is one of the most important activities for good health. During sleep, your body cycles through four different stages. These all impact your brain waves, muscle relaxation, recovery, and breathing.

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Illustration by Brittany England

It’s no secret that sleep is one of the most important activities for good health. When we sleep, our bodies take time to:

  • repair muscles
  • grow bones
  • manage hormones
  • sort memories

Sleep can be broadly segmented into rapid eye movement (REM) sleep and non-REM (NREM) sleep. Most adults will enter sleep from the drowsy state into NREM sleep.

NREM sleep is divided into three sub-stages: stage N1, stage N2, and stage N3. Older classification had four stages of NREM sleep. In the current rules, NREM stage 3 and NREM stage 4 are combined as stage N3.

Sleep stages occur in cycles lasting 90 to 120 minutes each. Four to five cycles occur during a typical night of sleep. Shifting of stages occurs over the course of the night, typically with an increased percentage of NREM sleep in the first half of the night and an increased percentage of REM sleep in the second half of the night.

In this article, we’ll explore these stages of sleep, discuss sleep disorders, plus tips for getting better sleep.

  • Non-REM sleep happens first and includes three stages. The last stage of non-REM sleep is when you sleep deeply. It’s hard to wake up from this stage of sleep.
  • REM sleep happens about an hour to an hour and a half after falling asleep. REM sleep is when you tend to have vivid dreams.

NREM Stage N1

This stage of non-REM sleep is the typical transition from wakefulness to sleep and generally lasts only a few minutes. Stage N1 is the lightest stage of sleep; patients awakened from it usually don’t perceive that they were actually asleep

During this stage:

  • Eye movements are typically slow and rolling.
  • heartbeat and breathing slow down
  • muscles begin to relax
  • you produce low amplitude mixed frequencies waves in the theta range (4 to 7 Hz)

NREM Stage N2

This next stage of non-REM sleep comprises the largest percentage of total sleep time and is considered a lighter stage of sleep from which you can be awakened easily. This is the stage before you enter deep sleep.

During this stage:

  • heartbeat and breathing slow down further
  • no eye movements
  • body temperature drops
  • Sleep spindles and K-complexes are two distinct brain wave features that appear for the first time

NREM Stage N3

This final stage of non-REM sleep is the deepest sleep stage. Stage N3 sleep is known as slow-wave, or delta, sleep. Your body performs a variety of important health-promoting tasks in this final non-REM stage.

During this stage:

  • arousal from sleep is difficult
  • heartbeat and breathing are at their slowest rate
  • no eye movements
  • body is fully relaxed
  • delta brain waves are present
  • tissue repair and growth, and cell regeneration occurs
  • immune system strengthens

REM Stage R

There are two phases of REM sleep: phasic and tonic. Phasic REM sleep contains bursts of rapid eye movements, while tonic REM sleep does not.

Stage R occurs about 90 minutes after you fall asleep, and is the primary “dreaming” stage of sleep. Stage R sleep lasts roughly 10 minutes the first time, increasing with each REM cycle. The final cycle of stage R may last roughly between 30 to 60 minutes.

During this stage:

  • eye movements become rapid during phasic REM
  • breathing and heart rate increases and become more variable
  • muscles become paralyzed, but twitches may occur
  • brain activity is markedly increased

When you fall asleep at night, you cycle through all of these stages of sleep multiple times — roughly every 90 minutes or so.

For something so necessary to our health and well-being, there’s still so much we don’t know about sleep. But here are seven fun facts that we do know:

  1. Human beings spend 1/3 of their lives sleeping, while cats spend roughly 2/3 of theirs asleep. Other animals, like koalas and bats, can sleep up to 22 hours a day.
  2. Newborn babies need roughly 14 to 17 hours of sleep per day, while teenagers require about 8 to 10 hours each night. Most adults need 7 to 9 hours of sleep.
  3. Sleep deprivation can have a hugely negative impact on health. Even as little as 24 hours without sleep can cause significant mood swings, difficulty functioning, and altered perception.
  4. Energy levels naturally dip at two distinct times of the day: 12-2 p.m. and 8-9 p.m. This explains the post-lunch fatigue that some people feel during the middle of the day.
  5. Higher altitudes can have a negative impact on sleep quality. According to research, this may be because of a reduced amount of slow-wave (deep) sleep.
  6. Although there’s still much to learn about sleep, the biggest thing we know is that sleep is just as crucial to good health as nutrition and exercise.

According to the American Academy of Sleep Medicine, the number of people annually who experience a sleep disorder is about 70 million. Sleep disorders can have a negative impact on sleep quality, which in turn can lead to other health problems. Below, you’ll find some of the most common sleep disorders and how they’re treated.


Insomnia is a chronic sleep condition characterized by difficulty sleeping. Some people have trouble falling asleep, others are unable to stay asleep, and some have trouble with both. Insomnia often causes excessive daytime sleepiness and fatigue.

Cognitive behavioral therapy (CBT) is the primary treatment for insomnia. CBT may also be combined with sleep medications, which are able to help people fall and stay asleep. For some people, improving sleep hygiene may also help.

Sleep apnea

Obstructive sleep apnea (OSA) is a condition in which the body stops breathing during sleep. These periods of no breathing, called apnea, happen because the airways of the throat become too narrow to allow air flow. Like insomnia, this condition can negatively affect sleep quality.

The first line of treatment for OSA is a continuous positive airway pressure (CPAP) machine. The CPAP creates enough airflow to allow a person with sleep apnea to breathe properly during sleep.

If the CPAP doesn’t help, bilevel positive airway pressure (BiPAP or BPAP) may be considered to help patients tolerate the pressures. In some cases, an oral appliance or surgery may be necessary for OSA.

Shop all Healthline-approved products for snoring and sleep apnea in our sleep shop.

Restless leg syndrome

Restless leg syndrome (RLS) is a neurological disorder that causes an uncomfortable feeling in the legs, which occurs in the evening as bedtime approaches and when the individual is resting or trying to fall asleep. People with RLS often have trouble getting enough sleep because of their symptoms.

Certain medications are FDA approved to help manage RLS symptoms. Practicing good sleep hygiene can also help relax the body before bed and make it easier to fall asleep.

Shift work disorder

Shift work disorder is a condition that commonly affects those who work outside of a regular 9-to-5 schedule. This disorder can cause an imbalance in the natural circadian rhythm, or sleep-wake cycle. People with this disorder are at a higher risk for increased daytime sleepiness and health issues.

Treatment for shift work disorder includes strategic napping, avoiding stimulants like light at the correct time, and, if possible, reducing the number of hours worked. For people who sleep during the day, it can also help to use light-blocking tools like eye shades or curtains.


Narcolepsy is a chronic central nervous system disorder that causes extreme daytime sleepiness with “sleep attacks” along with poor sleep at night. Type I narcolepsy also causes cataplexy, which is a sudden, physical collapse caused by loss of muscle control.

People with both type I and type II narcolepsy often experience extreme disruptions in their daily life.

Medications like stimulants and SSRIs are used to treat the symptoms of narcolepsy. At-home treatments, like good sleep hygiene, diet and exercising regularly, can help to promote healthful sleep.

Dealing with narcolepsy can be challenging. Making adjustments in your daily schedule may help which include strategic napping. Support groups and counseling can help you and your loved ones cope with narcolepsy.

Practicing good sleep hygiene is the best way to get quality sleep at night. Here are some ways you can improve your sleep hygiene:

  • Spend time outside in the sun during the day. Exposing your body to natural light during the day can help maintain a healthy circadian rhythm.
  • Exercise or move your body throughout the day. Getting in at least one exercise or movement session each day is a great way to improve your sleep quality.
  • Limit your nap time to no more than 20-30 minutes. There are benefits to napping. But if you nap for longer than 30 minutes, it can leave you wide awake when it’s finally time for bed.
  • Avoid stimulants and certain foods before bed. Caffeine, nicotine, or alcohol before bed can interrupt your sleep, as can foods that cause indigestion or stomach upset.
  • Limit your screen time an hour before sleeping. TVs, phones, and other electronic devices emit blue light, which can interrupt the hormones that help you fall asleep.
  • Create a comfortable bedroom environment. Investing in a high-quality mattress, pillow, blanket, and other relaxing bedroom items, can help you sleep better. Want suggestions? Browse our market, filled with editor-trusted and expert-verified pillow and mattress recommendations.

Incorporating these tips slowly over time can greatly improve your sleep quality. But if you’re still having trouble falling or staying asleep, it may be time to visit a doctor to discuss more options.

Your body cycles through the stages of sleep each night: three stages of non-REM sleep and one stage of REM sleep. During these sleep cycles, our breathing, heart rate, muscles, and brain waves are all affected differently.

Getting enough sleep is important for health-promoting activities like digestion, growth, and memory. Certain sleep disorders, like insomnia, can cause poor sleep quality and difficulty functioning throughout the day.

The best thing you can do to improve your sleep quality is to address any underlying conditions and work on your sleep hygiene.