4 Different Stages of Sleep: How Sleep Cycles Work?

How Sleep Cycles Work

When we wake up, we don’t remember most of what happened throughout the night, except for some vivid dreams that have occurred during the time. It’s no wonder sleep is shrouded in mystery! Because of the nature of sleep, we are lead to believe that it’s a passive state – a time when the brain shuts down almost completely. But, sleep is an active state!

The myth of it being a passive, unconscious state was debunked only recently, in the 1950s. Science has revealed that the process is vital to the rejuvenation of the body and mind. There are four stages of sleep. Each is different from the rest. All of them complete one sleep cycle.

Each sleep cycle lasts about ninety minutes. Humans go through five sleep cycles on average every night. Each stage has its unique purpose, so to say. Those restorative functions include memory consolidation, hormone regulation, and muscle recovery. The first three sleep stages comprise the NREM (non-rapid eye movement) period. The fourth stage is REM (rapid eye movement) sleep.

Don’t worry; it’s not as intimidating as it sounds. We’ve broken down all the confusing jargon for you. First, let’s see what the stages of sleep are.

Stage One

Sleep CyclesIt’s known as the transitional phase. It’s when your mind begins to drift off while you still feel somewhat awake, and the muscles start to relax. During this phase, you float in and out of consciousness. It is the lightest stage.

It’s not uncommon for one to experience sudden muscle spasms. A falling sensation often accompanies them. The feeling can jolt you back into wakefulness, but just for a moment. The drowsiness eventually ends, things wind down, and you move onto the second phase.

Stage Two

This stage lasts almost half of the time spent asleep. It is also light. The core body temperature decreases, and the heart rate begins to slow. You are not aroused or awakened as easily as in the previous phase.

The brain waves slow down, and eye movement stops. However, there are occasional, specific bursts of rapid activity. Those bursts of waves are also known as sleep spindles. They are accompanied by sleep structures called K complexes. It is thought that their role is to protect the brain from awakening. 

Stage Three (Deep Sleep)

The most restorative stage. Until recently, researches though there were 5 stages of sleep. But, new research has shown that stages three and four are the same. This phase is also known as the deep stage.

It’s when its hardest to wake you up. If you do wake someone up during this phase, they will probably be groggy and disoriented for a couple of minutes.

This is also the period of SWS (slow-wave sleep), which is still an NREM phase. It’s called that because the brain waves slow down even more. That frequency is known as delta. There’s still the occasional faster wave.

As this phase progresses, the frequency of faster waves decreases, while the frequency of delta waves increases. Breathing slows down and becomes deeper, as well as more rhythmic. Blood pressure drops even further.

During SWS, the body becomes immobile, and there is no muscle movement. However, the muscles are still able to function. That is the phase when children might experience sleepwalking, bedwetting, and nightmares. That happens with adults as well, but not as often. If you hear someone talking in their sleep, you’ll know they are at number three.

Deep Sleep

SWS is very rejuvenating to the body. It is when the boy releases hormones that help regulate appetite and growth. The growth hormones rejuvenate exerted tissues and muscles. Appetite controlling hormones prevent you from feeling too hungry during the day.

Both of these types of hormones are vital to your development. Blood flow to the muscles increases during SWS. The increased blood flow ensures your muscles are getting the restorative oxygen and nutrients they need.

Stage Four (REM)

This is the only REM phase. That’s why it’s also called the rem cycle, even though it’s not an entire cycle, but a part of one. That is when the brain starts to burst with activity. The average adult spends approximately 20% of sleep in REM. An average baby, on the other hand, spends about 50%.

woman in deep sleepWe call it REM (rapid eye movement) because its when the eyes start to dart all over. The muscles and limbs are paralyzed, only temporarily, of course. During NREM (non-rapid eye movement), the body heals while the mind rests. But, during REM, the body takes a break while the mind energizes itself.

Breathing is irregular and shallow. The blood pressure and the heart rate rise. The body loses some of its ability to regulate its temperature. And, the sweetest part? This is when dreams happen!

Dreams are the outcome of desynchronized and heightened brain waves. The frequency is almost the same as when you are awake. This is a key to sharp and alert daytime functioning, as it revitalizes the brain.

As this phase comes to an end, you begin to wake up. To prepare you for the day ahead, your core body temperature starts to rise.

If someone or something wakes you up before you complete the REM stage, you might experience grogginess. The feeling may last several minutes or several hours. You might feel a heightened sensation of sleepiness for up to four hours.

You can dream four to six times each night. Everyone dreams. Even when you don’t remember your dreams, you dream.

You remember your dreams better if someone wakes you up during REM. Scientists think that muscle immobility (also known as muscle paralysis or muscle atonia) occurs during this stage so that we wouldn’t act out our dreams. Due to the lack of muscle tone, obstructive sleep apnea is the worst during this period. Also, sleep talking can happen during this stage of sleep.

REM was discovered relatively recently, in 1953, almost as soon as machines for monitoring brain activity were invented.  Some ideas were floating around, which claimed that the deprivation of REM leads to insanity. Scientists have disproved them since them.

Research has shown that the lack of REM may help alleviate clinical depression. The exact reasons behind that are still unknown. Research also shows that there is a link between REM and memory and learning.  

Sleep Cycles

The progression through different stages of sleep is known as a sleep cycle. On average, a person goes through four to five cycles per night. Every 90 to 120 minutes, a new cycle beginsInterestingly enough, you don’t go directly from phase three to REM. The cycle goes back from deep to light sleep, ending with the REM phase. Then, it all starts over with light sleep.

In most adults, a cycle begins with stage one, which is the shortest. Phase two comprises about 50% of total sleep time, while phase three comprises between 5% to 15%.

The first cycle is also the shortest. It’s because the first REM period is short. It begins ninety minutes after falling asleep. As the night progresses, every next REM period lasts longer. The average cycle lasts anywhere between 100 and 120 minutes.

man in a Deep Sleep

The light phase lasts a bit longer in the second cycle. As indicated before, the REM lasts longer too. Stage three lasts long, but not as long as in the first cycle. In the third cycle, stages one and two last even longer. Stage three lasts a short while, and the REM lasts longer. In every cycle after this, your body alternates between REM and light stages until you wake up.

Changes With Age

How you slumber changes throughout your life – from infancy, through toddlerhood, school age, and adolescence, to adulthood. Newborns (from birth to 4 months) – During this age, there are no distinctive sleep stages. Slumber is characterized as quiet, active, and indeterminate. The quiet phase corresponds to non-REM, while the active period corresponds to REM. The active phase is significantly longer than the quiet phase.

The active stage allows for frequent awakening or arousals. It must be so because of the regular periods of feeding during the first days of a baby’s life. Infants ( 4 months to 7 y.o.) – The different stages become more apparent in infancy.

Sleeping routines can be developed, and slumber becomes more consolidated. Infants typically have two to three daytime naps. During the night, they are asleep for ten to thirteen hours.

  • Toddlers: (1 y.o to 3 y.o.) – Sleeping patterns are fully developed now. Stage three amounts to about 25% of the total time they are asleep. They sleep nine and a half to ten and a half hours per day. Naps are reduced to one nap per day, which usually takes place in the afternoon. If they nap later in the day, it can disrupt the deep stage at night.
  • Preschoolers (3 y.o. to 6 y.o.) – Like with toddlers, preschoolers spend an average of nine to ten hours a day sleeping. Naps become less common.
  • School-age (6 y.o. to 12 y.o.) – Almost everything remains unchanged during this age, except for the naps. They become even less frequent.
  • Adolescents (12 y.o. and older) – The average adolescent spends nine to nine and a half hours a day sleeping. However, the onset takes place a bit later due to the physiological changes in the circadian rhythm.

Because of these changes, many adolescents want to stay up late more often but also want sleep “for just five more minutes” in the morning. It’s not just because they want to avoid school; it’s nature! So, if you have a teenager, cut them some slack and allow them to sleep in a bit.

As you become an adult, the circadian rhythm shifts again. Six and a half to eight hours of sleep per night become normal. But, as you know, the desire to “sleep in” every morning never disappears.


Q: How long is a sleep cycle?

A: The first cycle lasts about ninety minutes. Every cycle that comes after, lasts 100 minutes to two hours.

Q: How to get more rem sleep?

A: The jury is still out on this one. So far, scientists say that for longer REM periods, it’s best to exercise regularly and avoid alcohol and stress.

Q: In what stage of sleep do you dream?

A: Dreams occur during REM.

Q: How long is a REM cycle?

A: The first REM period lasts only for ten minutes. With each cycle, the REM period increases. The last REM period lasts up to an hour.

Q: How many stages of sleep are there?

A: There are four stages in total.

Q: How much rem sleep should you get?

A: We don’t know what’s the optimal number of REM stages per night. One thing is for sure; they are good for you. If someone wakes you up in the middle of a REM stage, you’ll experience grogginess in the next several minutes or the next several hours. That is why it is important to know how many hours of sleep should you need for the proper rest.


Getting enough shut-eye is essential to health. Scientists are pretty clear on that. And, even though stage three is considered to be the most important, you need the other three as well. In these hectic and stressful times we live in, the world of dreams often suffers. As a consequence, our lives suffer. Don’t fall into that vicious cycle.

Make sure you are on good terms with the Sandman! If you wake up tired every morning, it may be a good time to get a new pillow, try a high-quality mattress, or talk to a doctor. Everyone deserves a restful night, every night.

If you found this topic interesting, you could read our other articles such as why do people yawn, or maybe you are interested to find out how to overcome the fear of sleep.