Sleep Physiology Overview Vector

Sleep Physiology: Overview and Practice Questions

by | Updated: Jul 10, 2024

According to, over 50 million Americans suffer from sleep disorders, underlining the essential role sleep plays in our health and well-being.

This article provides an overview of sleep physiology—the study of the processes our bodies undergo during sleep, including stages such as REM and non-REM sleep, and their impact on our health.

It will provide an overview of how sleep affects cognitive functions, emotional well-being, metabolic health, and the immune system, with a focus on disorders like sleep apnea.

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What is Sleep Physiology?

Sleep physiology refers to the study of the processes and mechanisms that govern the sleep cycle, including the physical and biochemical events that occur in the body during sleep. It encompasses understanding the various stages of sleep—primarily REM (Rapid Eye Movement) sleep and non-REM sleep—and how these stages cycle throughout the night.

Sleep physiology test infographic vector

Each stage of sleep serves different functions, from physical restoration and memory consolidation to emotion regulation and cognitive maintenance.

The field of sleep physiology also explores how external factors, such as light exposure, diet, and stress, can affect sleep patterns and quality. Moreover, it investigates the impact of sleep on overall health, including its influence on the immune system, metabolic rate, cardiovascular health, and mental well-being.

Sleep disorders, such as insomnia, sleep apnea, narcolepsy, and restless legs syndrome, are a key area of study within sleep physiology.

Researchers and clinicians study these conditions to understand their causes, how they disrupt normal sleep physiology, and their effects on physical and psychological health.

Note: The goal of sleep physiology research is to develop effective treatments and interventions for sleep disorders and to enhance the quality of sleep for better health outcomes.

Stages of Sleep

The stages of sleep are divided into two main categories: Non-Rapid Eye Movement (NREM) sleep and Rapid Eye Movement (REM) sleep.

Each stage has distinct characteristics and plays a crucial role in overall health and well-being. The sleep cycle progresses through these stages in a cyclic manner, typically repeating every 90 to 110 minutes.

Here’s a breakdown of each stage:

NREM Sleep

NREM sleep accounts for about 75-80% of total sleep time and is divided into three stages:

  • N1 (Stage 1): This is the lightest stage of sleep, acting as the transition from wakefulness to sleep. It lasts for about 1-5 minutes and muscle tone throughout the body begins to relax. Brain wave activity slows down from the awake state, characterized by theta waves. It’s easy to wake someone up from this stage.
  • N2 (Stage 2): This stage represents deeper sleep as the person becomes more disengaged from the surroundings. It lasts for about 10-25 minutes during the first cycle, and the duration increases with each cycle. Heart rate and breathing rate are regular, and body temperature drops. Brain waves continue to slow but are marked by brief bursts of electrical activity known as sleep spindles and K-complexes. It accounts for about 45-55% of total sleep in adults.
  • N3 (Stage 3): Commonly known as deep or slow-wave sleep (SWS), this stage is crucial for physical recovery and growth, immune system strengthening, and memory consolidation. It’s the hardest stage from which to be awakened. Brain waves are at their slowest, with delta waves dominating. Initially, it lasts for 20-40 minutes, but the duration decreases through the night.

REM Sleep

REM sleep occurs about 90 minutes after falling asleep and lasts about 10 minutes in the first cycle, increasing in duration with each cycle, up to an hour.

It is characterized by rapid eye movements, increased brain activity, and vivid dreams. Muscle tone in the body is significantly reduced, a phenomenon known as REM atonia, to prevent acting out dreams.

Heart rate, breathing, and blood pressure are closer to waking levels but can vary widely.

Note: REM sleep is crucial for cognitive functions such as learning, memory processing, and emotional regulation.

The Sleep Cycle

Throughout the night, the brain cycles through these stages in a predictable pattern, moving from N1 to N2, N3, and then into REM sleep before starting again at N1.

This cycle repeats approximately every 90 to 110 minutes. The amount of time spent in each stage changes throughout the night, with more deep NREM sleep occurring in the first half of the night and an increase in REM sleep in the later half.

What is a Sleep Disorder?

A sleep disorder is a condition that disrupts normal sleep patterns, affecting the quality, timing, or duration of sleep. Common examples include insomnia, sleep apnea, narcolepsy, and restless leg syndrome. These disorders can significantly impact physical health, emotional well-being, and overall quality of life, necessitating medical attention and treatment.


Sleep disorders can be broadly categorized into several types, each affecting sleep in different ways.

Here are some of the most common types:

  • Insomnia: Difficulty falling asleep or staying asleep. This is one of the most common sleep disorders.
  • Sleep Apnea: Characterized by pauses in breathing or periods of shallow breathing during sleep. Obstructive sleep apnea is the most common type, where the airway becomes blocked or collapses during sleep.
  • Restless Legs Syndrome (RLS): A disorder that causes an uncontrollable urge to move the legs, usually due to an uncomfortable sensation. It often occurs in the evening or nighttime hours when you’re sitting or lying down.
  • Narcolepsy: A chronic sleep disorder that causes overwhelming daytime drowsiness and sudden attacks of sleep. People with narcolepsy often experience disturbed nighttime sleep and an abnormal sleep cycle.
  • Circadian Rhythm Sleep Disorders: These disorders involve persistent issues with the sleep-wake cycle. They may include conditions such as delayed sleep phase disorder, where individuals go to sleep and wake up much later than normal, and advanced sleep phase disorder, where individuals fall asleep and wake up much earlier than most people.
  • Parasomnias: This category includes sleep disorders that involve abnormal movements, behaviors, emotions, perceptions, and dreams. Examples include sleepwalking, night terrors, and REM sleep behavior disorder.
  • Sleep-Related Breathing Disorders: In addition to sleep apnea, this category includes conditions like central sleep apnea and sleep-related hypoventilation, where decreased breathing occurs due to a lack of respiratory effort or impaired airway function.

Note: These disorders can have significant impacts on health, mood, and overall quality of life, making diagnosis and treatment essential.

What is Sleep Apnea?

Sleep apnea is a serious sleep disorder characterized by repeated interruptions in breathing during sleep. These interruptions (i.e., periods of apnea) can occur when the throat muscles intermittently relax and block the airway during sleep (known as obstructive sleep apnea, the most common form), or when the brain doesn’t send proper signals to the muscles that control breathing (known as central sleep apnea).

People with sleep apnea often experience multiple episodes of breathing interruptions per night, leading to significantly disrupted sleep. This can result in excessive daytime sleepiness, morning headaches, irritability, and difficulty concentrating.

Long-term, untreated sleep apnea can increase the risk of cardiovascular problems, such as high blood pressure, heart attack, stroke, and diabetes.

Symptoms often include loud snoring, observed episodes of stopped breathing, abrupt awakenings with gasping or choking, and a sore throat upon waking.

Treatment options vary depending on the severity and type of sleep apnea but may include lifestyle changes, continuous positive airway pressure (CPAP) therapy, oral appliances, and surgery.

Types of Sleep Apnea

Sleep apnea can be primarily classified into two types, each with distinct causes and characteristics:

  1. Obstructive sleep apnea
  2. Central sleep apnea

Obstructive Sleep Apnea

Obstructive sleep apnea (OSA) is the most common form of sleep apnea and occurs when the muscles in the throat relax excessively during sleep, leading to a partial or complete obstruction of the airway.

When this happens, the individual’s breathing can stop for several seconds to over a minute, leading to a decrease in blood oxygen levels. The brain then signals the body to wake up just enough to reopen the airway, often with a loud gasp or snort.

This cycle can repeat many times throughout the night. Risk factors for OSA include obesity, aging, anatomical variations that reduce upper airway size, smoking, and alcohol use.

Central Sleep Apnea (CSA)

Less common than OSA, central sleep apnea (CSA) is characterized by a failure of the brain to properly control breathing during sleep.

Unlike OSA, CSA is not caused by a physical blockage of the airway but by the brain’s inability to signal the muscles to breathe due to instability in the respiratory control center.

This can result from various medical conditions, including heart failure, brain tumors, brain infections, and the use of certain medications.

CSA can occur with or without Cheyne-Stokes respiration, a pattern of cyclic changes in breathing associated with heart failure and stroke.

Summary: Both types of sleep apnea disrupt sleep and reduce its quality, leading to excessive daytime sleepiness and increasing the risk of cardiovascular and metabolic diseases. However, their treatments differ due to their distinct causes. OSA is commonly treated with lifestyle changes, noninvasive ventilation, oral appliances, or surgery to keep the airway open during sleep. In contrast, CSA might require treating the underlying condition or using adaptive servo-ventilation (ASV) devices, which adjust the pressure based on the person’s breathing patterns.

FAQs About Sleep Physiology

How Long Can You Go Without REM Sleep?

Extended periods without REM sleep can lead to significant psychological and physiological consequences, including irritability, cognitive deficits, and impaired memory.

While there’s no exact limit to how long one can go without REM sleep before these effects become severe, research suggests that REM sleep deprivation can have noticeable effects in as little as one night.

Chronic deprivation of REM sleep over days or weeks can exacerbate these symptoms, leading to significant mood disturbances and decreased cognitive performance.

Note: The body tends to rebound by increasing the proportion of REM sleep once normal sleep patterns are resumed, a phenomenon known as REM rebound.

What Stage of Sleep is the Hardest to Wake From?

The hardest stage to wake someone from is NREM Stage 3 (N3), often referred to as deep sleep or slow-wave sleep (SWS).

During this stage, the brain produces delta waves, which are the slowest and highest amplitude brain waves, indicative of the least responsive state of sleep to external stimuli.

Waking up from N3 sleep can result in grogginess and disorientation, a state known as sleep inertia.

What is the Difference Between Obstructive and Central Sleep Apnea?

Obstructive sleep apnea (OSA) and central sleep apnea (CSA) are two types of sleep apnea that differ primarily in their causes:

  • Obstructive Sleep Apnea (OSA): This is the more common form of sleep apnea and occurs when the throat muscles intermittently relax and block the airway during sleep. This leads to repetitive stopping and starting of breathing, snoring, and restless sleep.
  • Central Sleep Apnea (CSA): This form occurs when the brain fails to send proper signals to the muscles that control breathing, leading to periods of no breathing effort at all. Unlike OSA, CSA is not caused by a physical blockage of the airway but by a failure in the respiratory control center of the brain.

Note: Both conditions disrupt sleep and can lead to similar consequences, such as daytime fatigue, but they have different underlying mechanisms and often require different treatments.

What is Polysomnography?

Polysomnography, often known as a sleep study, is a comprehensive test used to diagnose sleep disorders.

This procedure records various physiological functions during sleep, including brain waves (EEG), eye movements, heart rate, breathing patterns, blood oxygen levels, muscle activity, and more.

Conducted typically in a sleep lab overnight, polysomnography helps identify conditions such as sleep apnea, restless leg syndrome, narcolepsy, and sleep-related behavioral issues by providing a detailed picture of an individual’s sleep cycle and stages.

Final Thoughts

Sleep physiology is fundamental in understanding the critical role sleep plays in our health and well-being. It examines the stages of sleep and their functions alongside the impact of sleep disorders like insomnia and sleep apnea.

Recognizing and treating these conditions is essential for improving overall health and well-being.

As studies advance, the significance of giving priority to sleep for overall well-being becomes increasingly clear, emphasizing the crucial role of sleep in maintaining a healthy lifestyle.

John Landry, BS, RRT

Written by:

John Landry, BS, RRT

John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.


  • Patel AK, Reddy V, Shumway KR, et al. Physiology, Sleep Stages. [Updated 2024 Jan 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024.
  • Karna B, Sankari A, Tatikonda G. Sleep Disorder. [Updated 2023 Jun 11]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024.

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