Stages Of Sleep: How Sleep Cycles Works

Animation of a boy sleeping - Sleep Standards

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Expert Verified By: Dr. Amita Fotedar, Ph.D

Overview

Sleep process is a whole lot more complicated, and it is a much more active state than you might think. From falling asleep to waking up, the sleeper’s body passes through 4 different stages of sleep. Typically, a sleep cycle consists of two distinct parts – NREM (non-rapid eye movement) sleep and REM (rapid eye movement) sleep. The body goes through this 4 -stage cycle 4 to 6 times over the course of the night, spending an average of 90 minutes in each stage.

Each stage of sleep offers myriad of benefits like; restorative function including, hormone regulation, muscle recovery, and memory consolidation.  The mind and body are deprived of vital elements without a full night of good sleep.

Stages of Sleep

Check out what happens in each stage of a typical sleep cycle, when the body falls asleep.

Stage 1 – Light Sleep

Stages of sleep | Bullet points explain stage 1 in stage of sleep

Stage 1 is the transitional phase or light stage sleep. It is the first stage of a typical sleep cycle and lasts 5 to 10 minutes on an average. It is one of the shortest stages of sleep and occurs when you find yourself floating in and out of consciousness.

During this NREM sleep stage, you might experience hypnic jerk (unexpected involuntary contraction of body muscles). People indulge in brief “catnaps” during this stage and the body starts to ‘slow down’, causing lethargy, drowsiness, hypnic jerk followed by a falling sensation that jolts you back into consciousness. Your sleep cycle slips you into stage 2 after winding down in stage 1.

Stage 2 – Intermediate Sleep

Stages of sleep | Bullet points explain stage 2 in stage of sleep

Stage 2 in a sleep cycle is also an NREM phase. About 50 percent of the total time duration of a sleep each night is spent in stage 2. In this stage, the core body temperature decreases and the heart rate begins to slow down.

During stage 2, brain waves slow down with the occasional increase in brain wave frequency termed as sleep spindles and the eye movement stops. If you were to schedule a “power nap” you would want to wake up after this stage of sleep. It is also characterized by the unstructured periods that oscillate between muscle relaxation and muscle tone.

Stage 3 – Deep Sleep (Slow-wave sleep)

Stages of sleep | Bullet points explain stage 3 in stage of sleep

The encephalic region produces slower delta waves during Stage 3, and that is why it is termed as stages of slow-wave sleep. Eye movement or muscle activity is absent these stages. If the sleeper is woken up during these stages of sleep, they will most likely feel groggy and disoriented for minutes after they awake.

Slow-wave sleep (SWS) is basically a NREM sleep stage. It is the deepest sleep that the body enters throughout the night. The number of delta waves increases and the faster waves decrease, as the body advances from stage 2 to stage 3.

Other experiences include:

  • Blood pressure drops even further
  • Breathing becomes deeper, slower, and more rhythmic
  • Hormones are released that aid in both growth and appetite control
  • The growth hormones replenish muscles and tissues that were exerted over the course of the day
  • No eye movement and the body becomes immobile
  • Rejuvenation of the body
  • The flows of blood to the muscles increases, offering vital nutrients and the restorative oxygen.
  • Children sometimes experience bedwetting, nightmares, and sleepwalking.

REM Sleep (Rapid Eye Movement Sleep)

Stages of sleep | Bullet points explain stage 4 in stage of sleep

REM sleep is different from other sleep phases because the brain is bursting with activity. Usually, this stage of sleep takes place 90 minutes after sleep onset. The duration of REM sleep is marked by substantial changes in the body’s physiology. These include:

  • Accelerated respiration
  • Faster heart rate and breathing
  • Increased brain activity
  • Increased blood pressure
  • Random/rapid movement of the eyes
  • Sexual arousal in both men and women
  • Limited thermoregulation, shivering or sweating
  • Arm and leg muscles become temporarily paralyzed (atonia)

The brain is very active during REM sleep, yet your arms and legs become temporarily paralyzed during this stage. The first stage of REM typically lasts 10 minutes, with each phase of this sleep recurring and getting progressively longer, and the final one that lasts for 1 hour.

For an individuals who do not suffer from any sleep disorder, respiration rate and heart rate become erratic and faster during REM sleep. The fingers, face, and legs might quiver. REM is very important for memory and regulation of emotions. REM is a mixture of brain (encephalic) states of commotion and muscular immobility. During this sleep, you are able to clear the brain of the things that are not needed there.

It is also that time of the sleep, where protein synthesis at the cellular level is at its peak. This ensures that the multiple processes in the body are functioning properly.

What is the sleep cycle?

Sleep Cycle | Sleep Standards
Sleep Cycle

A sleep cycle is the advancement through the various phases of non-REM sleep to REM sleep before beginning the progression again with Non-REM sleep. Typically, an individual would start a sleep cycle every 90-120 minutes resulting in 4 to 5 cycles per sleep time, or hours spent asleep.

One does not directly transition from non-REM sleep (deep sleep) to REM sleep. In the majority of the cases, a sleep cycle starts with a brief duration of Stage 1 sleep in which the body starts to enter the phase of relaxation and a sluggish state occurs with slow-rolling movement of eyes. Stage 1 is important as it enables the body to enter Stage 2; the first measurable stage of deep sleep (non-REM sleep), even though awakenings or arousals are frequent.

Stage 3 is typically found next in the progression, moving through the sleep cycle. This restorative stage lasts between 5-15 percent of total time most of the adults fall asleep. Stage 3 is longer in duration in case of children and pubescents.

REM sleep is “paradoxical” because of its similarities to wakefulness and starts about 90 minutes after you fall asleep. The first period of REM sleep is shorter in duration being the first REM period of the night and lasts for 10 minutes.

The sleep cycle process starts again, with time durations of Stage 1, 2 & 3 combined before reversing back to REM again for lengthier time durations, as sleep time continues.

FAQ: Why Do We Sleep?

When does Deep Sleep occur?

Deep sleep occurs in stage 3 in stages of sleep. This is a state of deep and restorative sleep known as delta sleep or slow-wave sleep.

This is the deepest stage of sleep. It is difficult to arouse the sleeper, and if he/she gets awakened, there will be feeling of disorientation for a few minutes. A lot of high-voltage slow-waves appear in EEG (a recording of the electrical activity of the brain), in this stage.

As people become older, their ability to get into deep sleep state declines. People who are less than 30 years old usually experience 2 hours of curative restorative sleep nightly while older adults may get just 30 minutes. You get less deep sleep as you get older.

When does REM Sleep (paradoxical sleep) occur?

Stages of sleep | Graph explain when REM sleep occur

A sleeper’s sleep time (6-8 hours for adults) can be divided into two parts.  The first part comprises mostly of Stages 2 and 3 in majority of the people with sporadic periods of Stage 1 and short paradoxical (REM) sleep periods.  When the night progresses, Stage 1 and 2 remain with longer time durations. There is more REM occurrence and Stage 3 starts to diminish.

A sleeper typically experiences 3 to 5 REM sleep periods in total sleep duration. The longest paradoxical sleep period is the time right before getting up for the day.  If the sleep is disrupted, and the sleeper is woken prematurely from completing the REM sleep duration, he can experience a period of sleep inertia in which a heightened sensation of sleepiness can happen for several minutes or even several hours.

In the REM sleep stage, eyes jerk rapidly, heart rate becomes faster, blood pressure increases, breathing becomes shallower and more irregular, and limb muscles paralyze temporarily. Brain waves during this stage are same as when the person is awake. Also, males experience erections and the body loses the capacity to regulate its temperature.

When does Dream occur?

Animation a girl sleep in bed with moon in background

Most vivid dreams occur during REM sleep state (also known as an active sleep state) as a result of increased brain activity. This is the reason; it is sometimes termed as paradoxical (desynchronized or activated) sleep.

During REM sleep, the encephalic waves are considered to be of low amplitude, and the activity is enhanced than that seen in Stages 2 and 3. 

An individual usually dreams 4 to 6 times every night. A study reports that when awakened from REM sleep, a person is 75 percent likely to remember his dreams in comparison to the 10 percent who remember dreams when woken during non-REM sleep.

REM sleep is mostly accompanied by muscle immobility (paralysis). This muscle paralysis or atonia occurs as a protective means to prevent people from physically “acting out” their dreams — a dangerous, rare, problem called REM sleep behavior disorder. Several scientific studies have reported that the reason for muscle paralysis during REM sleep could be our prevention from any injury while trying to act out our dreams.

What impacts Sleep stage distribution?

There are many factors that can have an impact on your sleep stage distribution.

Age

As we age, the variability between individuals increases. Small children have a totally different sleep stage distribution. Young people and teenagers need different sleep quantity because of their growth and development of their cognitive abilities.

Infancy and early childhood witness the highest percentage of REM sleep. The percentage starts declining during the pre-adulthood and young adulthood and diminishes further in older age. It has been observed in multiple cases, old aged people get into REM sleep more expeditiously and remain in this sleep for a longer duration.

In older people sleep efficiency is lower and in them sleep becomes more fragmented. Older people sleep early and get up earlier because in them, the circadian rhythm is moved a little forward. Getting sunlight and staying active at the right times helps set the circadian clock.

Previous nights

The amount of sleep you need relies on the amount of quality sleep which you have had recently since it is a homeostatic process. It is also dependent on the metabolism of the brain.  If the brain has been doing more and more activity, then you need more sleep.

If you have not slept well and are sleep deprived, you will often see a rebound in the quantity of deep sleep on the first night after sleep loss.

Circadian rhythms

Circadian rhythms are responsible for our sleep patterns. 

SCN (the suprachiasmatic nucleus or nuclei) or the body’s master clock, controls the release of melatonin, a hormone that activates sleep process.

The clock receives information from the optic nerves about incoming light, which forwards information from the eyes to the brain. In the night, when there is less light—the SCN enables the brain to produce more melatonin in order to increase drowsiness.

Temperature

Sleep is significantly impacted by temperature. Extreme temperatures like too much heat and too much cold can disrupt your stages of sleep especially REM sleep because the body’s tendency to thermoregulate during REM sleep stage is lower.

The human body switches back and forth between shivering as a warming mechanism and sweating as a cooling mechanism when external temperatures are not ideal.

National Sleep Foundation suggests that the ideal temperature for a good sleep should be between 15 – 20 degrees Celsius (60 – 67 degrees Fahrenheit)

Drugs

Prescription drugs that can suppress sleep include:

  • High BP drugs like beta-blockers
  • Steroids, including prednisone
  • Diet pills
  • Oral contraceptives
  • Attention deficit hyperactivity disorder stimulant medications
  • Inhaled respiratory drugs
  • Seizure medications
  • Some non-prescription drugs that can affect sleep distribution
  • Pseudoephedrine
  • Medications with caffeine
  • Illegal drugs such as cocaine
  • Nicotine, that can reduce total sleep time

A glass of wine may help you fall asleep faster but tends to lower the quality of sleep.

Alcohol prevents REM and deep sleep since it keeps you in the lighter stages of sleep.

Sleep disorders

Sleep problems like narcolepsy and sleep apnea can have an impact on the sleep cycle. Sleep disorders suppress both NREM and REM.

References

Medical News Today: What does it mean when we dream?

The Biology of REM Sleep.

Healthline: What Is Deep Sleep and Why Is It Important?

Sleep Cycle: How Sleep Cycle works

Brain Basics: Understanding Sleep

What is a sleep EEG?

Additional Resources:

Infographic: 22 Proven Tips That Help You Sleep Better

Sleep Deprivation: Symptoms, Effects, Treatments, & Prevention

Why Do We Sleep?

Best Pillow For Sleep Apnea

Best Mattress For Sciatica

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