Understanding the Science of Sleep: What Is the Sleep Cycle?

This blog post was written by Dr. Stephanie Liu (Dr. Mom)—Family Physician and Assistant Clinical Professor at the University of Alberta—for SleepDebt.ca, a website developed by Eisai Limited.

The quality of our sleep plays a crucial role in our overall well-being. It impacts not only our mental and physical health, but also how well we function while we are awake. While on the surface, sleep may seem like a simple act of closing your eyes and drifting off, sleep is incredibly complex and affects virtually all systems of our body! Sleep allows the brain to recuperate from the day and support the body's optimal functioning.

Unveiling the Sleep Cycle

Sleep follows a cyclical pattern, comprising stages of sleep that each serve a purpose. These stages include REM (Rapid Eye Movement) sleep and non-REM sleep. Each night, we transition through these stages in a repetitive manner, typically spanning across four to six sleep cycles of varying length. This intricate balance of sleep cycles is an essential aspect of our nightly rest, also heavily influencing our health and well-being during waking hours.

This image was adapted from The Sleep Foundation.

Non-REM Sleep—The Early Stages

Non-REM sleep can be divided into three stages:

NREM Stage 1 (N1):

When you initially drift off to sleep, you will enter NREM Stage 1, which can be thought of as being a transition between wakefulness and sleep. The N1 stage usually lasts just a few minutes. However, because you’re still in such a light sleep, it can be easier to wake you up.

NREM Stage 2 (N2):

After a few minutes of NREM Stage 1, you will drift into NREM Stage 2 sleep. While this is still considered a light phase of sleep, it’s deeper than N1 and this stage is characterized by a decrease in your heart rate and in your overall body temperature. During Stage N2, we see unique patterns of brain activity (called sleep spindles and K-complex). These patterns are important for our brains to process and consolidate memories from our waking hours.

NREM Stage 3 (N3):

This is a deeper stage of sleep from which it is more difficult to be disturbed. During the N3 stage of deeper sleep, your brain waves are slower, and when we observe brain activity over this time, we can identify ‘delta waves’ which are really important for restorative sleep. This stage of sleep is also known as slow wave sleep. While you are resting in this phase, your body is hard at work repairing and building tissues, bone and muscles. It’s also a crucial time for strengthening your immune system.

The REM Sleep Stage—Time to Dream!

As its name suggests, REM (rapid eye movement) sleep is characterized by rapid movements of your eyes behind your eyelids. You’ll likely enter your first period of REM sleep for the night around 90 minutes after you first fall asleep. During this time, we see increased brain activity and you may have some of your most vivid dreams. One clever function our bodies have developed during REM sleep is the temporary paralysis of our muscles which may be to stop us from physically acting out these vivid dreams. REM is about so much more than dreams though—this stage plays a pivotal role when it comes to our cognitive functions, learning and memory consolidation.

The REM Sleep Stage—Time to Dream!

The Balance of REM and Non-REM Sleep

If you’re getting a full eight hours of sleep (in an ideal world), you’ll likely go through four to six sleep cycles each night. Each one of these consists of a transition through all three stages of non-REM sleep, followed by REM sleep. A complete sleep cycle lasts around 90–110 minutes in total.

The structure and length of each sleep cycle can change throughout the night. Typically, in the earlier part of the night, you will spend more time in non-REM sleep, but towards the later part of the night, the REM stage can last longer. The sleep cycle can also vary a lot depending on your age. People over 60, for example, spend less time in REM sleep and their sleep cycles are shorter than younger adults.

The Balance of REM and Non-REM Sleep

The Sleep Cycle: An Intricate Balance

While the sleep cycle can vary (and indeed does, across the course of the night), it’s also an intricate balance that we rely on for sleep quality, memory consolidation, emotional processing, physical restoration and immune function. When the sleep cycle is disrupted (e.g., by alcohol, drugs, sleep disorders and other conditions that may cause sleep disturbances), this can have a big impact on our overall well-being and daily functioning.

The Sleep Cycle: An Intricate Balance

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