Four Stages of Sleep

It was once thought that sleep was a passive activity where the mind and body lay dormant. In reality, there is much more going on when we’re at rest. Nowadays, we know that sleep comes in stages, during which there are different types of brain activity.

The two main types of sleep the brain experiences are Non-Rapid Eye Movement (NREM) and Rapid Eye Movement (REM). NREM sleep is experienced first, consisting of the first three sleep stages; the fourth stage is where REM sleep occurs. REM sleep is considered the most restful and is also the type of sleep responsible for dreaming. The brain cycles through NREM and REM stages several times throughout the night.

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In this article, we will discuss the four stages of sleep and how they play a role in your overall quality of rest.

Stages of Sleep

When you first close your eyes and fall asleep, your brain enters the first sleep stage. From here, your brain waves get longer and slower as you drift deeper into sleep. Once you have reached REM sleep—the final sleep stage—your brain wave activity will increase again as you begin to dream. After this, your body will cycle through the last three stages of sleep—Stage 2, Stage 3, and REM sleep—repeatedly until it is time to wake up.

While there are four distinct stages, they can also be separated into two types—NREM and REM sleep. Stages 1, 2, and 3 of sleep make up NREM sleep, and Stage 4 of sleep is when REM sleep occurs.

The brain cycles through the NREM and REM sleep throughout the night, with REM cycles gradually getting longer as the night goes on.

Non-Rapid Eye Movement (NREM)

The name Non-Rapid Eye Movement (NREM) serves to differentiate this type of sleep from Rapid Eye Movement (REM) sleep, where the eyes move rapidly behind the eyelids as you rest.

In the progression of NREM sleep, brain waves become slower and more synchronized, and the eyes remain still. Adults spend far more time in NREM sleep than infants, who spend about half their time in REM sleep. By contrast, adults spend about 80% of their time in NREM sleep. The fact that adults naturally get less REM sleep than they did when they were infants highlights the importance of getting a full night’s rest in order to make the most of REM sleep.

Stage One

Stage one of your sleep cycle occurs right after you fall asleep, and only lasts for a short period of time—about 10 minutes on average. This is the only part of NREM sleep that doesn’t get repeated in the sleep cycle; unless your sleep is significantly interrupted and you fully wake up. People in stage one are generally easier to wake because the body and brain have not fully engaged in sleep activities.

Stage Two

During this stage, the body and mind are in a state of light sleep. Your heart rate will begin to slow and your core body temperature will decrease slightly. Your muscles will also begin to relax, which can help alleviate any tension from the prior day. The second stage of sleep lasts for about 20 minutes.

Stage Three

This stage is considered the “deep” part of NREM sleep. The brain waves emitted during this time are larger and slower than in previous stages, resulting in the term “slow-wave sleep” being used to describe this stage. This is also the most efficient stage for the restorative effects of sleep, where the body repairs tissues and bones and helps strengthen the immune system.

Rapid Eye Movement (REM)

This fourth and final stage of the sleep cycle is where dreaming occurs. Rapid Eye Movement (REM) sleep, whose name refers to the way your eyes quickly move behind your eyelids, is the part of sleep when your brain is most active.

When you dream, your brain activity closely resembles brain activity when you are awake. Due to a temporary paralysis implemented during REM sleep, signals from your brain don’t go to your limbs and appendages. This is done by your brain to keep you from “acting out” your dreams.

It can take a while to reach the REM sleep stage—about 90 minutes after you have first fallen asleep. At the end of this stage, the brain enters a “light sleep” state as it did before in the second stage, and the cycle continues. As the night goes on, the REM stage of the cycle gets progressively longer. In order to receive the full benefits of REM sleep, it’s important to make sure you’re sleeping long enough to fully experience it.

Cycles During Sleep

Despite naming sleep stages “Stage 1, Stage 2, Stage 3, and Stage 4,” we don’t actually cycle through those stages in that particular order after completing the first sleep cycle. Misleading, right? Let’s talk about how our bodies and brains really cycle through sleep stages to clear up any confusion.

The first sleep cycle, beginning when your head hits the pillow, is the only sleep cycle when you’ll experience Stage 1 of sleep—this is because the first sleep stage is very light sleep, so you’re not likely to reach this sleep stage again unless something disturbs your rest and awakens you. And even then, if the wake-up is brief enough, your body may fall right back into Stage 2 of sleep without skipping a beat.

Once you progress from very light sleep to the deeper stages of sleep, you’re fully at rest and hard to awaken.

After the first bout of REM sleep, your body restarts sleep at Stage 2 and cycles through Stages 2, 3, and REM the rest of the night. These stages of sleep keep you in semi-deep and deep sleep all night. The duration of each stage fluctuates as the night goes on, but REM sleep consistently gets longer with each sleep cycle.

How Much Time Should I Spend Asleep?

How much sleep time we need varies from person to person, but most adults should aim for 7 to 9 hours of rest.

Your first sleep cycle will likely cover 70 to 100 minutes, while later sleep cycles should last 90 to 120 minutes. Your body cycles through the different stages about five times every night.

Our sleep calculator can help you determine your ideal bedtime. You simply count back from when you want to wake up, and factor in the time you need to fall asleep.

How Do Sleep Disorders Affect My Sleep Cycle?

Disorders such as insomnia and obstructive sleep apnea can prevent you from fulfilling a complete sleep cycle. A chronic lack of sleep is tied to decreased daily performance, such as reduced cognition, increased stress, and a lowered immune system response.

The effects of insomnia are relatively straightforward. When it takes you longer to fall asleep or you have difficulty staying asleep, you spend less time in the deep sleep stages. Many recovery tasks occur only in these deeper sleep stages, so you feel at less than your best when you wake up.

Sleep apnea is more complicated. The disorder impacts your sleep stage dynamics, “manifesting as shorter bouts and increased number of stage transitions.” This fragmented sleep is a key part of why sleep apnea patients often suffer from daytime sleepiness.

Conclusion

Quality sleep is experienced in cycles, and that cycle consists of both NREM and REM sleep stages. Both of these have their own functions and aspects, largely differentiated by brainwave activity. As sleep progresses through the night, the body performs a “housekeeping” routine, keeping your muscles and tissues healthy. It is important to experience a full night’s rest, as it takes the body time to make its way through the stages of sleep and their cycles.