Stages Of Sleep – What To Know About Non-Rem and REM Sleep Cycles

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Last Updated: Thu, February 14, 2019

Scientifically, sleep is considered to be a state of altered brain activity that is distinct from the waking state. During sleep, the brain cells tend to work more slowly, but quite intensively. Sleep is a crucial part of your everyday routine and you will spend approximately one-third of your life sleeping. Quality sleep is equally important as eating healthy and being physically active.

Even though sleep is necessary for every individual, its biological purpose is still a mystery. Sleep affects almost every cell and system in the human body including the heart, brain, lungs, and body functions such as metabolism, and immune function. Scientific research shows that lack of sleep or poor quality sleep increases the risk of diseases such as diabetes, obesity, cardiovascular conditions, and high blood pressure. Sleep is a complicated and dynamic process that ultimately affects how human being functions, and scientists are finally starting to understand it.

What You Need to Know About Sleep Stages

Many people always think that not much goes on when one is sleeping in their bed. This is mainly because people who are sleeping usually look pretty peaceful irrespective of whether they are sleeping at night or during the day. Individuals may talk or even move during their sleep but it is mostly never meaningful or even purposeful. However, when it comes to sleep, there is more that goes on than what we imagine.

As you sleep, your brain will move through several different stages. Each of these stages will have different characteristics that may not be visible at the eye level but become more apparent when a sleeping individual is hooked up to some scientific measuring devices such as an EEG, a device that records the brain activity. Variations in the brain’s electrical activity will show whenever every stage of sleep starts and ends.

During the different sleep stages, there are also physical body changes such as muscular tension and changes in eye movement. Healthy people typically pass through five sleep stages – first, second, third, fourth and REM sleep stage. These stages always progress cyclically from stage one through REM and then begin again with stage one. An average complete sleep cycle takes from 90 to 110 minutes, and each stage can last from 5 to 15 minutes. The first sleep cycles typically have shorter rapid eye movement sleeps, and more extended periods of deep sleep. Later in the night, REM periods start to lengthen and deep sleep decreases. Even though you may not be aware of it, your body is always busy even when the mind is turned off.


The first stage is light sleep, and it typically involves the act of falling asleep. You can feel extremely sleepy and may have challenges keeping your eyes open even for a minute. Additionally, your body may feel as though you are drifting in and out of sleep for some time. At this stage, you can be awakened even by the slightest noise and you will feel as though you have not got any rest.

At stage one, your body starts to prepare itself for deeper sleep. Your brain waves will begin to slow down, although they will look the same as when you were still awake. The breathing and heart rates will both go down, your muscles will relax, and your body temperature will start to fall. You will also become less aware of all external stimuli and your consciousness will start withdrawing from reality.

In stage one, many people experience something that can be described as the sensation of suddenly falling. This happens because the muscles are still active and they may suddenly contract. Every day you spend about 10% of your night in stage 1 or awake and some individuals tend to twitch at this stage. This first stage of sleep will usually last between 13 to 17 minutes. Basically, the first stage of sleep is when you fall asleep and it will only occur once during an uninterrupted night of sleep.


Finding an exact line between stage 1 and stage 2 can be quite a challenge. However, stage 2 usually starts about 10 minutes after stage 1. In the second stage of sleep, eye movement completely stops and brain waves become significantly slower. This is the time when the body prepares for deep sleep. The body’s temperature slowly begins to decrease, muscles relax further, and your heart rate slows down. The electrical activity in your brain will occur at a lower frequency compared to when you are awake.

Most of your sleeping hours are spent in stage 2 as you cycle through all 5 stages several times through the entire night. Stage 2 will normally take about half of the total time that is spent sleeping. Waking up at this stage is pretty easy. However, it is not as easy when compared to stage 1. Stages 1 and 2 are usually referred to as the light-sleep phase. Together they usually last about 20 to 30 minutes.


Stage 3 is the initial stage of deep sleep. In this stage, slow brain waves also known as delta waves become dispersed with smaller, faster waves. The shorter waves that tend to disappear as the sleep becomes deeper and deeper. Only in deep sleep, a person may experience parasomnias such as sleepwalking, night terrors, sleep talking, and bedwetting. Parasomnias typically occur during the transitions between non-REM and REM sleep.

Your breathing and heartbeat slow down to the lowest level during this stage and the muscles are completely relaxed. It is incredibly challenging to wake up stage 3 sleepers and they may not respond to noises of even 100 decibels. Waking someone at this stage is like trying to wake up an animal that is hibernating in the middle of winter. When they wake up at this stage, they will be confused, disoriented, and groggy.

People tend to reach stage 3 about 20 to 30 minutes after they start sleeping. At this stage, you are completely disconnected from reality. Stage three is one of the most restful parts of your sleep every night.


Stage 4 of sleep is basically a deeper extension of stage 3. Deep sleep continues, and the brain produces more delta waves than in stage 3. The difference between these two stages is the fact that stage 4 is usually characterized by longer and slower delta waves in the brain. At this stage, the body is usually slowed down than ever before. The breathing and heart rates are substantially different from what they actually are during the day. Individuals who have any sleep disturbances will start experiencing them as the body transitions from stage 4 into the REM sleep. In case you are awakened from this state, you will feel very disoriented for a few minutes.

Stage 4 of sleep usually starts approximately 45 minutes after you start sleeping. Stages 3 and 4 are called the deep-sleep phase and they usually constitute about 20% of the entire sleeping time. However, this proportion tends to decrease as one gets older.

REM Sleep Characteristics

The REM sleep cycle begins at least 80 to 100 minutes after you fall asleep. The deep-sleep phase ends abruptly and this is usually accompanied by a change in the sleeping position. The sleeping individual will switch back to stage 2 for several minutes before the EEG graph shows an abrupt change within a span of a few seconds. This is an indication of the REM sleep stage beginning.

In REM sleep, your brain waves activity is very similar to the activity in the state of wakefulness. The eyes are closed but more rapidly from side to side. This is the stage when intense dreaming occurs. Only 20% of the entire sleeping hours are spent on this stage. This percentage tends to be significantly higher in small children and infants.

When healthy individuals are in a state of REM sleep, their body muscles are deeply relaxed. If the muscles were not relaxed, those people may act out their dreams and this can have severe consequences. This is the main reason why the brain will put the body in this deep state of relaxation that is known as astonia and it borders close to paralysis. However, individuals who suffer from medical conditions, for example, Parkinson’s disease do not experience REM astonia and as a result, they can act out their dreams.

Why is REM Sleep Important?

Of all the sleep stages, however, the REM stage seems to have the most fascinating benefits. Just like the deep sleep stage, REM sleep has a healing power – experts say that REM sleep is a restorative stage where the body gets to heal itself.

REM sleep stimulates the brain – especially for children, REM sleep is vital in stimulating areas of their little brains. This brain stimulation is very important in retaining memories and also recording and learning new things. Just like deep sleep, REM sleep boosts the production of proteins. Scientists have approved that protein synthesis happens when the body is in sleep mode; something that is highly beneficial for muscle and overall body growth. Old or young, we all need to grow and develop our muscles.

REM sleep has been associated with pain relief – believe it or not, patients who, say, have a leg injury and get good sleep sessions will get some pain relief, while those that do not get REM sleep at all will often complain of the pain worsening, especially during the night.

Stage 1 Stage 2 Stage 3 Stage 4
Eyes are open, and responsive to all types of external stimuli. The person can hold comprehensible conversation. Brain waves are very similar to the state of wakefulness. Most vivid dreams happen in stage R. Body does not move. Transition between waking and sleep. When awakened in this stage, a person will claim the never fell asleep. Main stage of light sleep. In this cycle memory consolidation and synaptic pruning occurs. Slow brain waves on EEG readings. Slow brain waves on EEG readings.
16 to 18 hours per day 90 to 120 min/night 4 to 7 hours per night

What Is a Sleep Cycle?

A sleep cycle is a period of time required to go through the 5 stages of sleep explained above. It’s a very interesting fact that we don’t have to transition from deep sleep to rapid eye movement right away. During a sleep cycle, we can go through the stages of non-REM sleep. For example, you can go through light and deep sleep, and then reverse back from deep to light sleep, and end your sleep cycle in REM sleep. Of course, in order to wake up naturally, you have to go into the stage of light sleep. When you go back into light sleep, you don’t have to wake up necessarily, you have just completed one full cycle, and as mentioned above, a person goes through one than one sleep cycle per night. Once you have fallen asleep, you will be longer in deep sleep than in REM sleep. However, as the night progresses, you will spend more time in REM sleep and less in deep sleep.

It’s an interesting fact that sleep quality tends to change with the transition from one stage of sleep to another. Every transition of sleep is marked by subtle variations in bodily functions and every stage is part of a predictable cycle whose intervals can be observed. Sleep stages can be examined and monitored clinically using polysomnography that provides information about muscular and electrical states during sleep.

The Purpose of Sleep Cycles

Every sleep phase has specific purposes for the body. The main function of both light-sleep and deep-sleep is rejuvenating and restoring the body and the mind. During the REM stage of sleep, the brain is usually just as active as it is when one is awake. For you to process the memories and experiences of the previous day, both REM sleep and deep sleep are important.

The brain uses sleep time to weigh up the information that you have absorbed during the day while you were awake and to organize your memories. Additionally, the brain will also store any crucial information in the long-term memory and discard some other details and information. This is the main reason why having a good night’s sleep is important for your focus, memory and concentration.

How Long Is a Sleep Cycle?

The length of a sleep cycle can vary from person to person. Typically, a sleep cycle lasts from 90 to 120 minutes. The first cycle usually lasts around 90 minutes. After that, the sequences extend a bit and last from 100 to 120 minutes. A healthy individual goes through four to five sleep cycles a night.

What Is The Deepest Stage of Sleep?

The third and fourth stage of sleep is known as deep sleep. Deep sleep can also be called slow wave or delta sleep. It’s very difficult to wake up someone from stage 3 and stage 4. Children are nearly impossible to wake up from deep sleep, and in this stage, parasomnias such as night terrors, sleepwalking, sleep talking, and bedwetting may occur. In stage 3 and 4, there is no eye movement or muscle activity.

Deep sleep provides the most restorative sleep of all the sleep stages. This is why taking a short nap during the day probably won’t affect your ability to fall asleep at night. However, if you nap for too long and transition into deep sleep, you will have more difficulties hitting the hay at night. Deep sleep reduces your need for rest.

In delta sleep, human growth hormone is released which helps to restore your muscles and body. Your immune system also restores itself. Lastly, our body and mind refresh themselves for new experiences and learning the following day.

When Does REM Sleep Occur?

Deep sleep comes in the first half of the night, therefore, REM comes in the second half. This stage typically begins about 90 minutes after you first fall asleep, and the first cycle only lasts for 10 minutes. As the night progresses, REM cycles will become longer, and the last rapid eye movement will last up to one hour. Most healthy people experience 3 to 5 stages of REM sleep per night.

After REM sleep you transition into light sleep, therefore it may be possible to wake up. However, even if you wake up, you probably won’t remember it. Only if the waking period is long enough, you may remember it in the morning.

In REM sleep, brain waves in this stage are very similar to the ones in a state of wakefulness. During this sleep cycle, your breathing will become faster, irregular and shallow; your eyes will jerk rapidly, and your limb muscles will be paralyzed temporarily. Your heart rate increases and blood pressure rises. Males may develop erections. Lastly, your body loses some of the ability to regulate temperature.

When Do We Dream?

The brain is most active in REM sleep, therefore, this is the time most vivid dreams occur. When someone is awakened during rapid eye movement, he or she can remember their dream. REM stage is followed by temporary muscle paralysis. Many scientists believe this is a form of protection from acting out our dreams. Individuals who suffer from REM sleep behavior disorder, don’t experience this temporary muscle paralysis and therefore acts out their dreams. Even though most us don’t remember it, studies show that we all dream multiple times a night. A person dreams around 4 to 6 times every night.

Brain Waves During Sleep

Sleep research is still a relatively new and young field. An interesting fact is that scientists discovered REM in the middle of the twentieth century when new machines for monitoring brain activity were developed. Before this, scientists believed that sleep was a rather passive state when all brain activity ceased. Since 1953, researchers have disproved the idea lack of REM sleep leads to insanity. On the contrary, they actually found out that REM sleep can relieve the symptoms of clinical depression. New studies link REM sleep to learning and memory.

Does Sleep Cycle Change with Age?

The amount of time you spend in each stage of sleep depends on your age. Infants spend almost 50% of their time in REM sleep, while 50% of adults spend half of their sleep time in light sleep. 20% of adults spend most of their sleep time in REM. Older adults spend significantly less time in REM sleep, and that amount of time further decreases as they age.

Sleep Cycle in Babies and Children

The cycles of deep and light sleep in babies usually lasts about 30 to 50 minutes and then increases gradually in length across childhood. Some infants and children tend to fall deeply asleep quite quickly. Other children will sleep lightly while muttering and fidgeting for at least 20 minutes before they actually get into deep sleep.

Children normally wake up briefly before the end of every sleep cycle. All children do this because it is a customary part of healthy sleep. Some kids may even call out when they wake up at night and they will require assistance settling again. However, individual sleepers usually put themselves back to sleep. Not all parents will hear their children when they wake up at night.

Sleep Cycle in Adolescents and Adults

In adolescents and adults, every sleep cycle will end in a brief awakening and this can occur several times throughout the night. These awakenings do not usually interfere with our sleep and most times, we are not aware of them. If things do not change, then we will most likely go straight back to sleep. However, if anything changes around us, for example, if the pillow is missing, you feel anxious or uncomfortable, or you are disturbed by any noise, then one may wake up fully.

Factors that Affect Sleep Cycles

The sleep cycle is quite variable and it can be influenced by different agents. Sleep cycles, duration, quality, and the onset of sleep can be affected by sleep disorders. Additionally, the progression of sleep cycle can also be affected by sleep deprivation, stress, environment, and changing sleeping schedules frequently. A sleep disorder such as narcolepsy can affect the time it takes a person to achieve the rapid eye movement latency.

Psychological conditions such as depression can also shorten the length of the rapid eye movement sleep stage. Treatment modalities for psychiatric conditions also affect sleep in patients and can also induce a change in their sleeping habits. For example, the use of antidepressants such as Prozac can cause insomnia and trouble sleeping, and can also inhibit REM sleep stages.

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A wannabe journalist who somehow ended up as an art historian. She is a gamer, a coffee addict and a sleep aficionado. When she is not researching about sleep and finding out new ways to fight off the insomnia beast, she's spending time with her friends, gaming or visiting local museums.

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