What are the Stages of Sleep? The 4 Essential Sleep Cycles Explained

There are four distinct essential stages of sleep (also known as sleep cycles). You’ll learn what they are, how they work and why we need them.

Updated: May 28, 2020

In this in-depth guide, you’re going to learn about the stages of sleep and why they are important.

The stages of sleep break down into three main categories:

  • Wakefulness
  • Non-rapid eye movement sleep (NREM).
  • Rapid eye movement deep sleep (REM).

NREM sleep is further broken down into 3 stages: N1 (stage one), N2 (stage two), and N3 (stage three).

All the stages of sleep combine to create a sleeping cycle that lasts from 90 minutes to 120 minutes. An individual goes through 4-5 cycles per night.

Wakefulness

Wakefulness is a phase we can most accurately describe as individuals since we experience it in our most alert state. Open eyes, active muscles, and generally responsive behavior define this phase.

Illustration of wakefulness.

All speech is comprehensible during this time. We spend about 75% of the day in this stage. An objective measure of wakefulness is the beta waves our brains produce.

Stage 1 (NREM 1)

The first phase is light sleep or what is simply called “falling asleep.” It’s best characterized as the time when we are relaxing, but not yet asleep. During this phase, metabolism slows down. Muscles relax, eye movements slow down, and brain wave activity dips.

Illustration of stage 1 NREM 1.

The brain waves in this phase are theta waves. Theta waves show mind disengagement and relaxation. This state facilitates new ideas, so you may wake up ready to work on a new project.

If you’ve ever jolted awake, you’ve experienced a stage 1 “hypnic jerk.” Stage 1 is at the outermost layer of sleep, making it very easy to wake someone up. It is easier to wake someone in Non-REM sleep 1 than any of the other phases.

Stage 2 (NREM 2)

The second phase of sleep is more difficult to interrupt than stage 1. During this stage, activity within the body and brain decreases. This includes eye movements coming to a halt, brain waves slowing, muscles relaxing, body temperature lowering, and the heart rate decreasing.

Illustration of Stage 2 NREM 2.

Theta brain waves continue during stage 2, with intermittent bursts of activity called “sleep spindles.” These occur along with brainwave patterns that are known as “K complexes.”

Experts believe that spindles and K complexes protect us from disturbances during stage 2.

This is the stage of not-yet deep sleep where memory consolidation and synaptic pruning happens. It’s the main part of light sleep, and we spend 40-60% of our total night’s sleep here.

Stage 3 (NREM 3)

This is deep sleep. It is the phase that is the most beneficial. Marked by its restorative properties, relaxed muscles, and slow brain waves, this is the deepest stage before the rapid eye movement (REM) stage.

Illustration of Stage 3 NREM 3.

It’s very difficult to wake someone in this phase (as we see in people who sleepwalk and sleep talk). We spend about 5-15% of sleep in this phase, on average.

Even though we spend a relatively short time in this phase, it has the greatest effect on our bodies. If you get through the third phase of a sleeping cycle during a nap, you’ll likely feel alert when it’s actually time for bed.

That’s because your body already got a chance to restore itself a bit during the nap. In this regard, deep sleep is more restorative than REM sleep.

Did you know that, at this stage, the brain is emitting delta waves that indicate how little brain activity is happening as we sleep.

Stage 4, Rapid Eye Movement (REM sleep)

The last phase in the cycle is the REM sleep stage. It’s the phase most people recognize by name.

Rapid side-to-side eye movements, quick shallow breaths, increased heart rate, and increased blood pressure characterize REM sleep. Our bodies also lose control over body temperature regulation during REM sleep.

Illustration of Stage 4 NREM 4.

REM sleep, the deepest stage, is also the phase where our bodies are the closest to wakefulness behavior.

In fact, brainwaves during REM and wakefulness are similar. Thus, REM mimics light sleep without actually being light sleep

It’s easy to wake someone in REM sleep, but it’s best to avoid it. Waking up before finishing the REM stage results in grogginess. That grogginess can from minutes to hours. This is the phase where dreams, often very vivid, can happen… as well as lucid dreams.

Brain Waves During each Stage of Sleep

Our brains do a lot of work during sleep. Brainwaves go through a cycle of their own during each stage. There are four main types of brainwaves: alpha, beta, delta, and theta.

We cycle through different types of brainwaves as we cycle through different activities— including sleep.

Illustration of brain waves.

Beta waves occur when the brain is the most active and engaged in an activity. They are the fastest moving waves, having low amplitudes. Beta waves cycle 15 to 40 times per second.

Alpha waves occur during relaxed wakefulness. These brain waves tend to increase in people who participate in relaxing pastimes like meditation, nature walks, and similar activities. The brain’s alpha waves cycle 9 to 14 times per second.

Delta waves are the slowest of all waves, having the highest amplitude. When you have sleep with no dreams, delta brain waves cycle 2 to 3 times per second. There is very little brain activity with delta waves.

Theta waves occur when we disengage the most. Any activity that allows you to go into “autopilot” mode is likely accompanied by theta waves. They usually facilitate creative thought flow, day dreams, and other mentally disengaged things that require a mostly uninhibited state.

Surprisingly, REM sleep is not associated with only one type of brain wave. During REM sleep, brain activity is classified as “low voltage” and “random.”

Sleep Cycle Disrupters

If sleep could happen in a vacuum, it would benefit everyone. But it doesn’t. Thus, it’s important to recognize common sleep cycle disruptors.

Photo of a guy on a plane trying to sleep.

Any number of things can be disruptive to your sleep cycle. If you’re trying to get to the bottom of your sleep issues, these are a good place to start:

  • Light
  • Jet lag and shift work
  • Medical conditions
  • Medications
  • Caffeine/stimulants

Externally, each of these disrupters comes with their own unique symptoms. Internally, though, they have something in common. At least one factor involved with these sleep disrupters interrupts the cycle. This interruption prevents us from reaching deep REM sleep.

Light plays a factor in a couple of these. It is responsible for controlling our internal clock. Some of the cells in our retinas are sensitive to light. When light hits those cells, it sends a signal to the brain about whether it’s day or night. That’s why napping during the day is so difficult for some people.

Irregular sleep hours are also a factor. The amount of light outside tells your internal clock what it’s supposed to do. Irregular sleep hours militate against those signals.

Any medical condition that causes pain or discomfort is a potential disrupter. This includes anxiety, depression, menstrual symptoms, and more. Feelings of pain and discomfort often prevent us from reaching the stages of deep sleep that are restorative for as long as our bodies need.

Medications and caffeine can exacerbate poor sleep quality, too. They can affect heart rate, emotional stability, and more.

Whether their purpose is to keep you awake or make you feel better, if they cause you to stay awake longer than the standard 12-18 hours, they’re a sleep disrupter. This means less deep sleep and less REM sleep.

Does Your Sleep Cycle Change with Age?

Yes. As we age, our sleeping cycle changes along with our bodies. This is to accommodate growth and development.

Photo of an old man.

Newborn cycles are more centered around feeding schedules. Infant sleep patterns begin to get more complex as they adapt to sleep and feeding routines. By 12 months, the sleeping cycle is stable.

From 1 to 12 years of age, the sleeping cycle remains mostly the same. During these years, children sleep 9-11 hours per night, and spend most of the night in deep sleep or REM sleep.

One thing that changes as we age is the number of hours required to sleep. During the major developmental period between 1 and 12 years old, stage 3 is a crucial cycle.

Once you get to age 12, 9-11 hours is still the standard. A factor that does change, however, is the circadian rhythm. Adolescents sleep later and stay up later, but as they get older their sleep stages level out.

As an adult, sleep time typically hovers between 6.5 and 8 hours per night.

How to Reset Your Sleep Cycle

If your sleeping cycle is off and affecting you adversely, you can reset it. It’s not quite as easy as resetting an alarm clock, but here are a few tips to help you reset your sleeping cycle:


1. Go very slow

Illustration of an alarm clock.

It’s common to think that you can reset your sleep cycle by “just going to bed a few hours earlier.” This can actually make you feel worse. It’s similar to giving yourself jet lag. Instead of shocking your body, do it gradually.

Try going to bed 30 minutes to an hour earlier for 6 mornings. If you want to go earlier, increase the increment again for another 6 days.


2. Think in cycles

Illustration of a 24 hour cycle.

When you’re planning your new sleep schedule, think of your entire nightly sleeping cycle. Most people don’t do this.

Each cycle (with all of its sub stages) runs for at least 90 minutes.

So you need to plan your sleep hours the night before to get you through at least 4 full sleep cycles.


3. Use light to your advantage

Illustration of sunlight.

Our internal alarm clocks are light activated. You can use that to your advantage when you’re adjusting your sleep schedule. When it gets closer to your desired bedtime, start to dim the lights or wear blue light blocking glasses.

When you wake up, go to the part of your home where the most natural light is. It’s like retraining your body to respond to its natural rhythms.

After a while, you may simply start to fall asleep as the light changes.


4. Use assistance carefully

Illustration of sleep aids.

There are a few different things you can use for assistance. Melatonin, naps, and caffeine are popular tools. Use each of these with caution.

Taking melatonin too close to bedtime may leave you feeling groggy when you wake up.

A nap that is too long may cause you to feel too energized at bedtime. Caffeine has a similar effect. It’s all about timing when you’re hoping to increase your REM and deep sleep.


5. Fasting for 16-24 hours

Illustration of a plate of dinner.

Fasting has shown to delay the circadian rhythm, meaning if can possibly help reset your body clock, just by not eating.

That means no food whatsoever, for at least 16 hours. Only water.

Fasting can be incredibly difficult to accomplish for a lot of folks, so use with caution if it’s your first time.

If you need help tracking your fasting habits, use an app like Zero, which is designed for fasting and even has a circadian rhythm setting.

Sleep Cycle Apps to Track Stages of Sleep

The first step towards better sleep is understanding your cycle. Apps to track your sleep stages are a huge help.

Screenshot of sleep cycle apps. Track the stages of sleep.

Each of these apps uses your phone, your phone’s microphone, and a smartwatch (not required) to monitor your sleep. Here are few of the more popular apps:

But they all track movement, sounds, and more to let you know what quality of sleep you’re getting at each of your sleep stages. Sleep Cycle, Sleep Time, and Good Morning Alarm Clock have an alarm setting.

Choose your wake time and set the alarm through the app. That’s it.

It uses the information it gathers while you’re sleeping to wake you up during the lightest part of your sleep in order to maximize how refreshed you feel.