Understanding the Five Stages of Sleep

written by / May 26, 2019
Stages of Sleep - Featured

We’re all interested in and dependent on our sleep patterns. Of course, we are always trying to resist this fact. We try to spend more time working, learning, or having fun, all at the expense of the number of hours spent in bed. Unfortunately (or fortunately), we cannot deceive our bodies for too long.

Today, we’re going to take a few steps to understand the phenomenon of sleep, what the stages of sleep are, and why we need to get a normal night’s sleep!

What Is Sleep?

Sleep is a state of bodily rest prevalent throughout the animal kingdom. Although we know of some interrelations between the periods of sleep and important bodily processes, many of the aspects of sleep remain a mystery. Besides being a rest time, it is also a process with many phases, called sleep stages. Each stage plays a vital role for the human body.

Fortunately, the multitude of studies and information gathered on the process makes it possible to reach several conclusions. In this article, we will summarize what we know and what we assume about the role and importance of sleep and its stages for the body. To find out more about this vital process, you can have a look at our infographic which presents 80 eye-opening stats and facts about sleep.

How Do You Fall Asleep?

You may only know a little about the circadian rhythm, but you’re certainly familiar with the biological clock. The sleep cycle stages all relate to this circadian rhythm. The foreign word “circadian” comes from the Latin circa (meaning “around”) and diem/dies (which means “day by day”).

The circadian rhythm is a kind of biological clock that functions in the human body and can—ignoring external stimuli (day/night, time zone, ambient temperature, etc.)—regulate the onset of sleep and your overall sleep cycle time.

The anatomical background of the circadian clock mechanism is found in the hypothalamus (a part of the brain). It’s associated with the suppression of energy-consuming processes, lowering body temperature, and falling into a state of sleep.

The Role and Functions of Sleep

What’s known for sure about the stages of the sleep cycle? Sleep is closely related to the central nervous system. However, the physical regulation processes are not yet fully understood.

There are a few ways to further study sleep patterns. A sleep recording via polysomnography combined with electroencephalography (EEG), electromyography (EMG), electrocardiography (ECG), and many other parameters can be observed, depending on the nature and purpose of the sleep study.

How Many Stages of Sleep Are There?

Based on electrophysiological data, it has been proven that sleep includes the following three main stages:

  1. Wake (also called stage 0) – The EEG is present with the so-called alpha rhythm when a person has their eyes closed but is not yet asleep.
  2. NREM (non-REM, non-rapid eye movements) – This can be divided into sleep stages 1–4:
  • Stage 1 is a light sleep. Typically, there are rolling eye movements, less blinking, and a gradual decrease in muscle tension. At this stage, waking is easy.
  • Stage 2 is a transition stage. Eye movements are no longer possible, and the muscles relax even more.
  • Stages 3 and 4 include the stages of deep sleep. In this period, the threshold to actually wake up is the highest.

Characteristic features of NREM sleep are reduced heart rate and blood pressure, rhythmic breathing, and decreased muscle tension. One’s eye movements are slow, but they still “roll.” When roused from this phase, a person will often feel disoriented at first.

Since there’s non-REM sleep, what is REM sleep?

  1. REM (Rapid Eye Movement) (also called rapid-wave sleep or R-stage) – The results of the EEG performed during REM sleep resemble those obtained in stage 0 (wake stage). This phase is characterized by extreme shifts in the studied parameters: heart rate is accelerated and irregular, blood pressure is increased and unstable, breathing is also irregular, and the body can’t regulate its temperature. At the same time, the muscles are completely relaxed.

Why Is REM Sleep Important?

Stages of Sleep - REM and NREM

REM sleep is characterized by high brain activity and a relaxed body, whereas NREM sleep leads to an active body and a less active nervous system. During REM sleep, the body can’t sufficiently regulate certain critical biological functions. Because of this, people are more vulnerable to heart and respiratory disorders during this stage.

You might wonder what stage of sleep do you dream. Interestingly, almost all dreams happen in this stage, whereas they are rare in NREM stage. However, if a dream does present itself in the “deep sleep” stage, it is usually a nightmare.

Throughout the night, sleep goes through 4–5 cycles of 90–110 minutes. Each cycle contains consecutive phases: NREM is followed by REM sleep, and then a new cycle begins.

Different sleep cycles cover distinct time intervals, during which our nervous system changes rhythmically. NREM sleep occupies 75% of the entire duration of the cycle and is more pronounced during the first part of the night. In the last third of the night, REM sleep predominates. The latter lasts about 20–25% of one’s entire sleep time.

It’s also interesting to note that while deep sleep is considered a restorative sleep stage because it stores a person’s energy resources, REM sleep is essential for strengthening memory and preparing the mind for future situations.

Sleep and Age

It has now been confirmed that the distribution of both phases of sleep differs in different age groups. A newborn spends 16–18 hours asleep, and over 50% of that time is dominated by REM sleep. From ages two to five, this proportion transitions to 25% and then remains relatively stable even in adulthood. Therefore, we can predict the sleep cycle length by age.

One’s quality of sleep also varies based on one’s age. In early childhood, deep sleep is more prevalent. However, by the time we reach about 30 years of age, it becomes more common to wake up in the night.

The role and physiological significance of sleep remain a mystery to modern science. Nevertheless, its contribution to normal physical functions is undeniable according to psychological studies on the stages of sleep.

Sleep Stages and the Metabolism

It’s believed that NREM sleep is the anabolic (constructive) part of the entire sleep period. This phase is characterized by updating several of the body’s systems: nervous, muscular, immune, bone. However, in the case of sleep disorders, the first stage may be catabolic, which can impact one’s metabolism.

The reasons for this are different. REM sleep is associated with intense brain activity. It has been theorized that this is when our temporary memory becomes permanent, which is one of the leading benefits of REM sleep.

The Health Benefits of Sleep and How Much We Should Get

Adolescents and teenagers need at least eight and a half to nine and a half hours of sleep. For young and middle-aged people, this drops to between seven and eight hours. In the United States and some EU countries, many have adopted a 7.5-hour night. In Bulgaria, eight hours of sleep is considered the norm. There are also claims that seven hours and forty-five minutes are quite sufficient. The elderly should sleep eight hours or more to ensure that all sleep stages are present in the proper proportions during the night.

When we get a good night’s rest, both our mind and body can function better. From boosting our concentration to improving our immune system, there are numerous health benefits of sleep.

Sleep Disorders, Sleep Disturbance, and Sleep Debt

Insomnia is a sleep disorder that can be caused by quantitative or qualitative causes. When one can’t sleep long enough for complete bodily recovery, this disturbs the normal circadian rhythm. People who have this condition experience anxiety and wake up often, sometimes to the point where they can’t achieve all the phases and stages of sleep. Both too much REM sleep or a lack of REM sleep can negatively impact one’s health.

Typically, sleep disturbances cause a malfunction of one or more bodily systems: immune, nervous, endocrine, etc. Regardless of the disturbances that led to it, the longer a body is sleep deprived, the higher their sleep debt.

However, we can regulate any sleep disorder by planning our days a little bit better. It’s believed that an extraordinary sleep of 3–4 days, followed by a few weeks of a regular bedtime, is sufficient to restore the circadian rhythm and achieve a normal sleep cycle graph.

An effective sleep hygiene system requires that a person take certain actions to improve their sleep health. There are some essential elements that every person can control to protect their sleep and ensure a decent night’s recovery. For a healthy person to get the best sleep possible, they need to fulfill three crucial aspects: choose the right time to sleep, provide the proper environment, and maintain good sleep habits.


How many hours of deep sleep should I get?

According to the New Health Advisor, adults aged 18 and older need at least 1–2 hours of deep sleep per night. This accounts for about 20% of your total sleep. Some people, however, may find that they need more hours to feel completely rested. Keep in mind, it’s assumed that there is no such thing as too much deep sleep.

How much of your sleep should be REM?

Although there is no formal consensus on how much REM sleep you should get, we have to keep some facts in mind. Experts believe that dreaming helps you to process emotions and consolidate your memories. Dreams are most common at this stage. Therefore, you have to obtain enough REM sleep.

For most adults, REM takes up about 20–25% of their sleep. If someone gets excessive amounts of REM, they’re more likely to suffer from depression.

What stage of sleep makes you feel rested?

Stage 3 of the so-called non-REM sleep, is the period of deep sleep that you need to feel refreshed in the morning. It occurs more frequently during the first half of the night. Your heart rate and breathing slow down to their lowest levels during sleep, your muscles are relaxed, and it can be hard to wake you up.

How many sleep cycles should I get?

Each sleep cycle contains REM (rapid eye movement) sleep, but not necessarily deep sleep. The first stage of sleep is about 90 minutes, and adults usually have at least four or five sleep cycles per night, amounting to 6–9 hours of sleep.


If you get the recommended amount of sleep—seven to nine hours a night—you spend about a third of your life in bed. While this may seem like a lot, your mind and body are very busy during this time so you can be productive, energetic, and healthy when you’re awake. Being aware of the stages of sleep helps one ensure a better quality of sleep and thus, a better quality of life.

At medical school, it was easy to realize that sleep is crucial while studying during the many sleepless nights I spent. As a new mom, when the lack of sleep became even more evident, this was the real moment when I started appreciating the benefits of a good night’s sleep. Since sleeping is so essential to our health and immune system, I took it upon myself to educate people on different aspects of sleep by sharing the valuable information I have learned.