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Sleep: Quality, 5 Phases, Circadian Rhythm, Disorders

sleep circadian

By Jack Harley, MSc, University of Oxford 

When thinking about how much sleep we need, it’s common to focus on the number of hours of sleep we get. But while total sleep duration is important, it’s not the full story – the quality of our sleep also matters.

To understand sleep quality, it’s important to understand that sleep is not a uniform process. Rather, your brain and body go through several sleep cycles during the night, each cycle consisting of 5 unique stages. Each stage of sleep helps to ensure the mind and body wake up feeling refreshed.

Understanding the sleep stages can explain how certain sleep disorders such as insomnia may arise, and how you can improve your sleep.

 

What is the sleep cycle?

During the night, the brain and body progresses through a series of sleep cycles. Sleep cycles typically last between 90 minutes to two hours, and during this time the brain progresses through five phases:

Sleep stage Type of sleep Alternative names Typical length
Stage 1 non-REM  N1, light sleep 5-10 minutes
Stage 2 non-REM  N2, light sleep 10-60 minutes
Stages 3 and 4 non-REM  N3, slow-wave sleep, delta sleep 20-40 minutes
Stage 5 REM  REM sleep 10-60 minutes

A person will generally go through between 4 to 6 sleep cycles in a normal night of sleep lasting 7 to 9 hours. Sleep cycles vary from person to person and are influenced by a variety of factors such as age, alcohol consumption, and recent sleep patterns. During the night, you spend different amounts of time in each phase of sleep, and this changes in each sleep cycle. 

What are the stages of sleep?

There are 5 stages of sleep, 1 that involves rapid eye movement (REM), called REM sleep, and 4 that do not, known as non-REM (NREM) sleep. These stages are primarily determined by the recorded brain activity, which is distinct for each stage. The stages are listed below: 

Stage 1

The first stage of sleep is a non-REM sleep known as light sleep. This is essentially the ‘dozing off’ stage, as your eyes are closed but you may be easily awoken. During this stage, activity in the brain and body begins to decline, but the body has not yet fully relaxed. 

This stage typically lasts 5-10 minutes, and if the person is not woken up, they will quickly move into stage 2 sleep. During the rest of the night, the person will generally not spend any more time in stage 1 as they move into further sleep cycles. 

Stage 2

During the second stage of non-REM sleep, still considered light sleep, further relaxation occurs. The muscles relax, body temperature decreases, and breathing and heart rate slow. At the same time, eye movement stops and the brain shows a new pattern of activity. 

Brain activity slows down, although the EEG charts show short spikes of activity that actually prevent you from being easily woken up. These bursts of brain wave activity are also thought to also play a role in long-term memory consolidation. 

Stage 2 sleep lasts between 10 and 25 minutes during the first sleep cycle, becoming longer after each sleep cycle. During a night of sleep, the average person spends about 50% of their overall sleep time in stage 2. 

Stage 3 and 4

Stage 3 and 4 are known as deep sleep, and it is much harder to wake up someone if they are  in these stages. 

During these phases, muscles relax even further, and pulse and breathing slow. The brain produces a characteristic pattern called delta waves, which is why this phase is sometimes called delta sleep or slow-wave sleep (SWS). 

This phase of sleep is thought to play an important role in mental and physical recovery, as well as growth. Even though brain activity has slowed down, this stage of sleep contributes to creativity and memory. 

Most of the time we spend in deep sleep occurs in the first half of the night. During the early sleep cycles, deep sleep typically lasts 20-40 minutes. Later in the night, this type of sleep becomes shorter as you spend more time in REM sleep. 

Stage 5 

During stage 5, or REM sleep, brain activity increases to similar levels as seen during the day, and this is the phase of sleep in which dreams occur most frequently. 

During REM sleep, the muscles of the body experience a temporary paralysis known as atonia, with the exception of the eyes and breathing muscles. Although the eyes are closed, they move around rapidly, which is where the name ‘rapid eye movement’ (REM) sleep comes from. 

During REM sleep, heart rate and blood pressure increase, while breathing can become fast and shallow during this stage. REM sleep is believed to play an important role in cognitive functions such as memory and learning. 

You won’t normally enter REM sleep until you have been sleeping for at least 90 minutes. As you progress through each sleep cycle, the amount of time spent in REM sleep increases. 

While you may only spend a few minutes in REM sleep during the first sleep cycle, this may increase to an hour spent in REM in later cycles. Overall, adults spend roughly 25% of their total sleep time in REM sleep. As we age, the amount of time we spend in REM sleep tends to decline. 

 

Why do sleep stages matter?

The accumulation of time in each of the 5 stages of sleep is known as sleep architecture, and is important for allowing the brain and body to recover and grow. Regularly getting too little sleep, or not enough time in stage 3 to 5 may negatively impact important brain functions such as cognition, emotional processing and physical health. Sleep deprivation may also increase the risk of several health conditions such as type 2 diabetes and heart disease.

What affects the time spent in each sleep stage?

Several factors have been shown through research studies to impact sleep architecture, the amount of time spent in the 5 stages of sleep. These include: 

  • Age greatly affects the time spent in the stages of sleep. While newborns may spend around 50% of their total sleep time in REM sleep and enter this stage quickly, older adults may spend only 20% of the time in this stage.
  • Alcohol can reduce the amount of REM sleep early in the night. However, as the alcohol wears off, REM sleep rebound occurs resulting in more REM sleep later on. 
  • Sleep disorders such as obstructive sleep apnea or restless leg syndrome may cause you to frequently wake during the night, preventing you from getting enough deep and REM sleep. 
  • Recent sleep/wake cycles in previous days or weeks can affect the time spent in each stage of sleep during the night. 

Sleep disorders

Sleep disorders are a group of conditions that affect a person’s ability to regularly get high-quality sleep. While it is fairly normal to occasionally have a poor sleep, when this begins to occur on a regular basis it may indicate a sleep disorder. 

Types of sleep disorders include: 

  • Insomnia: is an inability to fall asleep easily or remain asleep during the night. Insomnia is a very common condition, experienced by up to 50% of people in the United States at some point in their lives.
  • Sleep apnea: involves temporary pauses in breathing during sleep. This may cause the person to wake up, or receive insufficient oxygen during the night. There are two types: obstructive sleep apnea, in which the airways are physically blocked, and central sleep apnea, in which the connection between the brain and the breathing muscles is not functioning properly.

What are the symptoms of sleep disorders?

Common symptoms of sleep disorders include: 

  • Fatigue during the day
  • Difficulty falling or staying asleep
  • Irritability
  • Anxiety
  • Difficulty concentrating
  • Weight gain


If you regularly experience one or more of the above, it is recommended that you consult your healthcare practitioner. 

How are sleep disorders diagnosed and treated?

To diagnose a sleep disorder, your doctor will generally perform a physical examination and take a medical history, and may request that a sleep study be conducted. Treatments for sleep disorders include pharmaceuticals or lifestyle changes, such as: 

  • Melatonin supplements
  • Sleeping pills
  • Exercise
  • Stress reduction
  • Limiting caffeine during the day
  • Reducing tobacco and alcohol consumption
  • Following a regular sleep schedule

Sleep and circadian rhythm

Just as our sleep follows a cycle, so too does our wakefulness and our transition from wakefulness to sleep. The 24-hour cycle that makes us feel sleepy at night and awake during the day is known as circadian rhythm, and is built into our biology.

Circadian rhythms control many physical and mental processes such as:

  • Sleep-wake cycle
  • Body temperature
  • Hormone levels 
  • Alertness and drowsiness

How does circadian rhythm work?

Different systems of the body follow circadian rhythms that are synchronized with a master clock in the brain. This master clock responds to a range of environmental signals to inform the body to sleep or be awake. Light is the most important signal for circadian rhythm, which is why exposure to bright lights from electronic screens at nighttime can alter our circadian rhythm. 

If we often feel tired during the day or awake at night, we may be experiencing a circadian rhythm sleep disorder.

What are circadian rhythm sleep disorders?

Circadian rhythm sleep disorders are a group of sleep disorders in which the circadian clock is altered. The most common types of circadian rhythm sleep disorders include: 

Delayed sleep phase syndrome (DSPS): you sleep and wake more than 2 hours later than what is typically considered a normal sleep-wake cycle. For example, you regularly go to sleep at 2 a.m. or later. People with this condition are often most alert and awake at nighttime, and it is most common in young adults. 

Advanced sleep phase syndrome (ASPS): causes you to go to sleep earlier than you want to (6 p.m. to 9 p.m.), and wake up in the early morning (2 a.m. to 5 a.m.). This condition is most common amongst older adults. 

Jet lag: your body’s circadian rhythm is synced to a different time zone. The sleep/wake cycle makes it hard to align your sleeping and waking times to the new environment. Jet lag can occur due to long flights, or changing time zones during travel. 

 

Shift work disorder: shift work may put your body’s circadian rhythm at odds with daylight hours. This may result in you getting up to 4 hours less sleep than the average person, as well as poor sleep quality.

What are the symptoms of circadian rhythm disorders?

Circadian rhythm disorders result in altered sleep onset, and often poor sleep, or daytime sleepiness. The most common symptoms include: 

  • Insomnia 
  • Daytime sleepiness
  • Stress
  • Reduced ability to focus
  • Sleep loss
  • Difficulty waking up

What causes circadian rhythm sleep disorders?

Circadian rhythm sleep disorders are caused by frequent disturbances in the circadian cycle, resulting in a disruption to the brain’s regulation of the sleep-wake cycle. In some cases, there may be a mismatch between the internal body clock and the external environment. 

Circadian rhythm sleep disorders may be caused by: 

  • Frequent changes in sleeping and waking times
  • Brain injuries such as stroke and dementia
  • Older age
  • Shift work
  • Jet lag

What is the diagnosis and treatment of circadian rhythm disorders?

To diagnose a circadian rhythm sleep disorder, your doctor will perform a physical examination, take a medical history, and may request for a sleep study to be performed. Treatment for circadian rhythm sleep disorders may include supplements or medications such as: 

  • Melatonin
  • Benzodiazepines such as Xanax
  • Nonbenzodiazepine hypnotics such as Ambien

Other lifestyle changes that may be prescribed to improve circadian rhythm include:

  • Avoiding eating a meal too close to bedtime
  • Exposure to bright lights (e.g. daylight) in the morning
  • Following a time-restricted eating schedule in which all food is consumed within an 8-12 hour period
  • Going to sleep and waking up at the same times each day
  • Eating breakfast at the same time each day

The above lifestyle recommendations have been proposed by Dr Satchin Panda of the Salk Institute and other circadian rhythm experts to improve circadian rhythm. 

Conclusion

Understanding the stages of sleep may help you to avoid sleep problems and excessive daytime sleepiness. Sleep occurs in cycles made up of five stages. Sleep disruption, especially during the deep sleep and REM stages, may result in poor sleep quality. 

To avoid sleep disturbance and short sleep duration, it is important to keep your circadian rhythm running in a healthy way. This will ensure you are not at an increased risk of a sleep disorder, including a circadian rhythm disorder. 

 

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