Stages of Sleep: What Happens in Sleep Cycle?

Many people believe that when they sleep, the brain “turns off” to rest. However, this isn’t always the case; there’s more going on behind closed eyes than one might think. Throughout the night, the brain goes through several different sleep cycles.

Starting with NREM (Non-Rapid Eye Movement) sleep and progressing to REM (Rapid Eye Movement) sleep, these cycles evolve through the various stages of sleep. Every 90 to 120 minutes, the average person begins a new sleep cycle, which means they will most likely go through four or five cycles during a night’s sleep.

We’ll go over the various stages of sleep and which are the most important, as well as how much sleep one should get and how sleep changes over time.

What Are the Stages of Sleep?

Sleep is divided into four stages: three NREM stages and one REM stage. The stages begin with light sleep and progress to a deeper state of sleep. When a sleep cycle is completed, the person will transition from REM to less deep stages of sleep, eventually returning to the lightest phase of sleep. Then the cycle starts all over again.

Sleep architecture refers to the division of a person’s sleep into different cycles and stages. A hypnogram can be used to visualise the sleep architecture of someone who has had a sleep study.

The American Academy of Sleep Medicine updated its sleep stage classification in 20074. (AASM). Most experts used to refer to five sleep stages, but the AASM definitions of the four stages now represent the majority of experts’ understanding of the sleep cycle.

Stage 1

Stage 1 sleep is the first stage of the sleep cycle. The body begins to relax and become drowsy at this point. Slow, rolling eye movements and the ability to be easily awakened are common signs of this phase.

As the body drifts to sleep, brainwave activity begins to slow down to theta brain waves. During this phase, some people may experience sudden jerks or muscle spasms, as well as the sensation of falling.

If the person is awakened during this time, they will most likely believe they were not sleeping at all. The first stage usually lasts about ten minutes.

Stage 2 

Stage 2 is the first phase in which the person is completely asleep, and it lasts longer than stage 1. During this time, people are less easily roused, and their slow-moving eye rolls usually come to a halt. Their body temperature will start to drop, and their heart rate will begin to slow.

As a person falls deeper into sleep, their brain waves will continue to slow. However, there are bursts of rapid activity known as sleep spindles during this phase. The brain waves slow down after a spindle occurs. Stage 2 accounts for about 40 to 60 percent of a person’s total sleep time.

Stage 3 

The body will enter stage 3 as it begins to move through sleep. This is a restorative stage that usually lasts less time than stage 2. For most adults, it accounts for five to fifteen percent of total sleep time. Children and teenagers, on the other hand, will typically spend much more time in this stage.

As the body prepares for REM sleep, a person’s brain waves slow down to become delta waves during this deep sleep. The body strengthens its immune system, repairs and regrows tissues, and builds bone and muscle during this time. Parasomnias such as sleepwalking, talking, or night terrors can also occur at this time.

It’s much more difficult for the body to wake up during stage 3 sleep, and if a person does, they’ll likely feel disoriented at first. This stage moves closer to REM sleep as the body produces more delta brain waves.

Patterns of REM Sleep

During REM sleep, brain activity increases to levels similar to those seen when you’re awake. At the same time, the body goes into atonia, which is a temporary muscle paralysis with two exceptions: the eyes and the breathing muscles. The eyes can be seen moving quickly even though they are closed, which is how this stage gets its name.

REM sleep is thought to be necessary for cognitive functions such as memory, learning, and creativity9. The significant increase in brain activity during REM sleep is responsible for the most vivid dreams. Dreams can happen at any stage of sleep, but they are less frequent and intense in the NREM stages.

Why Stages of Sleep is So Important?

The stages of sleep are important because they allow the brain and body to recover and develop. Insufficient sleep may explain some of the profound effects of insufficient sleep on thinking, emotions, and physical health if both deep and REM sleep is not obtained.

Sleepers who are frequently awoken during earlier stages of sleep, such as those suffering from sleep apnea, may find it difficult to cycle into these deeper stages. People who suffer from insomnia may not get enough total sleep to complete each stage.

What Affects Sleep Stages?

While there is a general pattern for sleep stages, there can be significant individual variation due to a variety of factors, including:

• Age: Over the course of a person’s life, the amount of time spent in each stage changes dramatically. Newborns spend far more time in REM sleep (around 50%) and may enter it as soon as they fall asleep. As they grow older, their sleep patterns begin to resemble those of adults, with most children reaching a comparable sleep architecture by the age of five. Elderly people, on the other hand, spend less time in REM sleep.
• Recent sleep patterns: An abnormal sleep cycle can develop when a person gets irregular or insufficient sleep for several days or more.
• Alcohol: Alcohol and other drugs can change the way you sleep. For example, early in the night, alcohol reduces REM sleep, but as the alcohol wears off, REM sleep rebounds, with longer REM stages.
• Sleep disorders: Sleep apnea, Restless Leg Syndrome (RLS), and other conditions that cause frequent awakenings can disrupt a normal sleep cycle.

When Does REM Sleep Take Place?

The first session of REM sleep lasts about 10 minutes and happens around 90 minutes after one falls asleep. The REM stages get longer as the person cycles through sleep, with the final stage lasting about an hour.

What Is the Recommended Amount of Deep Sleep?

According to studies, the average adult requires between 1.6 and 2.25 hours of deep sleep per night. Because deep sleep is so important for brain health and function, each person should spend 20 to 25 percent of their sleep time in this state.

Because their bodies are still developing, children and babies require more time in deep sleep. Children under the age of two require approximately 4.5 hours of deep sleep. Children aged two to twelve require slightly less time, around four hours. Teenagers and young adults require approximately three hours of deep sleep per night.

How Can You Improve Your Sleep Cycle?

While you may not have complete control over your sleep cycle, you can take steps to increase your chances of a healthy transition through each stage.

Focusing on improving your sleep hygiene, which refers to your sleeping environment (the best mattress, pillows, or sheets, etc.) and sleep-related habits, is an important first step. Getting more consistent sleep schedules, getting natural daylight exposure, avoiding alcohol before bedtime, and reducing noise and light disruptions can all help you get more restful sleep and promote proper circadian rhythm alignment.

If you experience excessive daytime sleepiness or suspect you may have a sleep disorder such as sleep apnea, it’s critical to speak with a doctor who can best guide your treatment.

Addressing underlying issues could lead to more restful and complete sleep cycles.