Stages of Sleep and the Sleep Cycle – What to Know About REM and Non-REM Sleep
It’s easy to think that there’s not much going on while you’re in bed. After all, sleeping people tend to look pretty peaceful, and in general, appear the same the whole time they are asleep. As a result, it’s easy to think about sleep as one extended but uniform activity.
In reality, there’s a lot more going on than meets the eye. The body and the brain move through several different, discernable stages while you sleep. Each stage has unique characteristics that, while not visible to the average person, become readily apparent when a person is hooked up to scientific measuring devices while they sleep.
These stages combine to form a sleep cycle, and each night when we sleep, we move through different stages, completing multiple sleep cycles. While knowledge about the details of sleep has grown dramatically thanks to new technology and focused research, there’s still a great deal that remains unknown about why we sleep in the way that we do.
In this guide, we’ll give an overview of what we know about the sleep cycle, each individual sleep stage, and what’s happening in our brain and body as the night progresses.
What is the Sleep Cycle?
The combination of the sleep stages forms a sleep cycle. In general, a sleep cycle lasts between 90 to 120 minutes, although the first nightly cycle is usually shorter, ranging between 70 and 100 minutes.
A full cycle consists of four stages. The first three stages make up non-REM (rapid eye movement) or NREM sleep. The fourth stage is REM sleep.
What Are the Sleep Stages That Make Up the Sleep Cycle?
Each stage plays a part in quality sleep, and the following sections give a more detailed explanation of what’s taking place during each phase of the sleep cycle.
Stage 1, also called N1, is the act of falling asleep. You may feel extremely sleepy or that you can’t possibly keep your eyes open for another minute. You may also feel like you drift in and out of sleep for a period of time. If someone wakes you up while you’re in this stage, you may not think that you got any rest at all.
However, Stage 1 involves your body preparing itself for deeper sleep. Your brain waves begin to slow, though they still look much like they did when you were awake. Heart and breathing rates both go down. You still have some eye movement, but not like you did when you were awake.
If you’ve ever jerked yourself awake thinking you were falling, you were in Stage 1 sleep. This happens because your muscles are still active during this stage, and sometimes they all contract together. It is easier to be disturbed or awoken during Stage N1 sleep.
It can be hard to find the exact line between Stage N1 and N2 sleep, but Stage 2 usually comes within 10 minutes after the onset of Stage 1. In this phase, your brain waves get even longer and more spread out, though these long-wave patterns are interspersed with flurries of activity.
We typically spend much more time in Stage 2 than Stage 1, especially as we go through multiple sleep cycles over the course of the night. It is estimated that on average, about 50% of sleep each night occurs during Stage 2. As people get older, they spend more and more time in this stage, with less time spent in REM sleep.
During Stage 2, your heart and lungs will get even more of a rest as heart rate and breathing slow even more. Eye movement ceases, and muscle contractions become fewer and farther between. It’s still pretty easy to wake up from Stage 2 sleep, but it’s not as easy as it was in Stage 1.
Stage N3 is known as deep sleep. In deep sleep, your brain begins to produce delta waves, which are slower brain waves. For this reason, this stage may also be known as “delta sleep” or “short-wave sleep.” During this phase, the body, like the brain, slows way down, leading some to believe that this phase is integral for physical recovery and conserving energy.
It is much more difficult to wake someone up when they are in Stage N3 sleep. Most people are so deeply asleep in Stage 3 that you have to shake them or touch them to wake them up. When they do wake up, they tend to be groggy, disoriented, or confused.
Stage 3 sleep is where some sleep disturbances begin. People who sleepwalk, talk in their sleep, struggle with wetting the bed, or have night terrors tend to have these issues during Stage 3.
During REM sleep, brain activity increases substantially with many more fast, alpha brain waves. Most dreaming, especially intense dreaming, takes place during REM sleep.
As the name implies, this stage involves significant movement of the eyes. In the rest of the body, there may be an increase in activity, but the muscles in the arms and legs remain motionless, which helps keep us from acting out dreams.
All of this sets REM apart from the other sleep stages. In total, REM sleep is believed to make up about 20-25% of our nightly sleep, although this can vary for any given individual or night.
Researchers believe that REM sleep plays an important role in brain function, although there is disagreement about the exact biological processes taking place. Some of the common theories are that this is a time when the brain does “housekeeping” to eliminate unnecessary information and consolidate memory.
How Do We Progress Through the Sleep Cycle?
While we often proceed stepwise through each sleep stage, that’s not always the case. Throughout the night, our sleep cycles are not consistent. For example, there may be brief N2 stages between stage N3 and REM sleep.
In addition, the first N3, deep sleep phase is usually the longest of the night. Later in the night, we usually spend less time in deep sleep. The amount of time we spend in each sleep cycle tends to be different depending on a number of factors including our age. The use of substances like alcohol can alter how we progress through the sleep stages as well.
Why Do Some Sources Mention a Five-Stage Sleep Cycle?
In 2007, experts settled on the current definition of the four-stage sleep cycle. Prior to that date, though, the sleep cycle was considered to have five total stages, four of which were NREM. Ultimately it was decided that there was insufficient divergence between the third and fourth NREM stages, resulting in the modern understanding of four stages: N1, N2, N3, and REM sleep.
Because the conception of the five-stage cycle was held for many years, you may still find references to it, especially in articles, studies, or other resources from 2007 or before.
While the scientific body of knowledge about sleep has grown considerably, we still don’t know exactly why our sleep reflects these stages and patterns of the sleep cycle. The more we learn about sleep, the more fascinating the study becomes.
There’s still so much we don’t know about why our brain waves change when we sleep, how sleep came to be, and what our brains are doing while we sleep. Researchers hope to continue building on our existing knowledge and with time be able to offer more insight that helps to answer these questions and allow us to optimize our nightly rest.