Understanding the Five Stages of Sleep
written by/ January 12, 2022
Putting off sleep is not a healthy habit, and most of us know it. What’s surprising, though, is that a long slumber doesn’t guarantee a day full of energy either.
While getting sufficient sleep is essential, it’s the duration and quality of each of the stages of sleep you enter that make for restful shut-eye. These cyclical changes of brain and body activity are the foundation of our well-being.
We’re far from being completely inactive during sleep, but what exactly are we going through? Why is each sleep phase important?
Find out all you need to know to upgrade your sleep below.
What Are the Stages of Sleep?
The physiological processes that take place during sleep perform three main functions—detoxification, restoration, and preservation. The electrical activity in our brains changes throughout our slumber, and the phases are defined by their most prevalent brain waves. These are easily identified through sleep activity recordings using:
- Polysomnography (PSG)
- Electroencephalography (EEG)
- Electromyography (EMG)
- Electrocardiography (ECG)
The variations in sleep waves by stages seen in these tests are universal for humans. The phases belong to one of two types of sleep:
- Rapid eye movement (REM) sleep
- Non-rapid eye movement (NREM) sleep
In essence, REM sleep is the complete opposite of NREM sleep. It is characterized by high brain activity and a relaxed (i.e., paralyzed) body, whereas NREM sleep is signified by an active body and less active brain.
The duration of either type of sleep changes during the night, and deep sleep gets shorter, while REM phases get longer.
The Sleep Cycle and Its Stages
During sleep, we enter one sleep stage after another (with some exceptions) and make the complete round in 90 minutes on average (between 70 and 110). As long as we’re asleep, we’re cycling through the stages.
This full rotation of sleeping phases (not counting wakefulness) is called a sleep or REM-NREM cycle. On average, a person experiences four to seven sleep cycles per night and needs between seven and nine hours of sleep each day to complete enough of them for restfulness.
How Many Stages of Sleep Are There?
Before 2007, sleep experts identified five different slumber stages—four in NREM sleep, plus the REM stage. The awake stage doesn’t count here.
However, based on recent electrophysiological data from different studies, the stages have been reclassified by the American Academy of Sleep Medicine (AASM).
Before, the NREM sleep stages 1–4 (formerly S1–S4) are now referred to as N1, N2, and N3. The REM stage is now dubbed stage R. NREM sleep is divided into three phases:
- Stage 1 (N1) is 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 (N2) is a transition stage. Eye movements are no longer possible, and the muscles relax even more.
- Stage 3 (N3) includes the stages of deep sleep. In this period, the threshold to wake up is the highest. In the former five stages of sleep in psychology, stage 3 was divided into two phases of deep sleep (stage 3 and stage 4).
During REM (Stage R) sleep, despite your muscles being relaxed (more precisely, paralyzed), you’re not resting at all! Your physiological parameters (heart rate, blood pressure, breathing) are unstable and change rapidly. What’s more, the brain is almost as active as when you’re awake.
Stages of Sleep: The Order
Sleep cycles cover different intervals during which our nervous system activity changes rhythmically.
NREM sleep occupies 75% of the entire sleep cycle duration and is more pronounced during the first part of the night. In the last third of the night, REM sleep is dominant. REM sleep accounts for 20% to 25% of one’s entire sleep time. The passage from one to another sleep stage is rarely perfect, and the phases are not always clearly distinct.
During a usual uninterrupted sleep, our sleep phase schedule looks like this:
Wakefulness → N1 → N2 → N3 → N2 → REM
What Happens During Each of the 5 Stages of Sleep in a Sleep Cycle?
Provided you’re not sleep-deprived, your sleepiness and alertness will be determined by your individual circadian rhythm. This internal rhythm is also known as a biological clock or chronotype.
It partly depends on external stimuli (the day and night cycle, time zone, ambient temperature, etc.) and regulates our overall sleep cycle time.
The anatomical background of the circadian clock is found in the hypothalamus (a part of the brain). When it’s time for sleeping, the hypothalamus suppresses energy-consuming processes, lowers the body temperature, and triggers sleep.
The wake stage (also known as stage 0, or wakefulness) is technically a part of the sleeping cycle, although—obviously—we’re not asleep during this time. Still, many people disregard the wake phase and say there are just 4 stages of sleep.
EEG testing during the wake stage shows different brain activity when our eyes are open and when they are closed:
- The open-eyed wake stage is dominated by beta brain waves, with a smaller presence of alpha waves.
- In closed-eye wakefulness, the alpha waves become prevalent.
Ideally, wakefulness is present only once throughout the night, and isn’t a part of the repeated stages of sleep cycles.
When more than 50% of the alpha brain waves your brain produces are transforming into low-amplitude mixed-frequency (LAMF) activity, you’re starting to fall asleep or enter the N1 sleep stage.
This so-called dozing-off stage (also known as the transitional phase) is the first one of the sleep cycle stages. In it, we’re becoming less aware of our surroundings.
That said, your comfort level plays a part in how fast you go through this stage. A good pillow, adequate mattress, or a high-quality weighted blanket can tuck you in, and shorten the time you need to get to the N2 phase.
If comfortable, the N1 phase is the shortest of them all, lasting between just one and five minutes. Still, even if your alertness is decreasing, the tone of your muscles is nearly the same as when awake, and you can be easily stirred from this stage.
Everyone has mildly different sleep cycles, so this sleep phase can last anywhere from 10 to 60 minutes.
The N2 stage is when:
- You become completely unaware of your surroundings
- Muscles relax
- Heart rate drops
- Breathing becomes regular
- Body temperature goes down
- Eye movement ceases
- Slow brain waves begin to appear.
Adults spend half of their slumber in the N2 phase, making it the lengthiest of the different stages of sleep. As shown above, usually, a sleeper revisits the N2 phase after N3 before going into REM sleep.
Other particularities of this stage are the so-called sleep spindles. These are sudden bursts of rapid brain waves in brain activity. Research has shown that sleep spindles during this sleep stage play an essential role in consolidating motor sequence memories (the procedural knowledge of a sequence of movements).
During this stage, the slow delta brain waves become dominant, and your muscles become even more relaxed. In other words, deep sleep starts here.
So, what happens in Stage 3 of sleep?
- Heart rate and blood pressure drop further
- The body begins to recuperate and regenerate physically
- Hormones boost body growth during this stage
- Your brain consolidates declarative memories (facts, stats, general knowledge, etc.)
The 20 to 40 minutes of Stage 3 sleep are considered the most important for restoring bodily functions and recovering from physical strain.
The most common sleep-related question is, “What Is REM Sleep?” The REM stage is also called paradoxical sleep because its EEG graph is similar to stage 0 (wakefulness).
The key characteristics of the REM stage are extreme shifts in physiological indicators, like:
- Accelerated and irregular heart rate
- Increased and unstable blood pressure
- Irregular breathing
- Body temperature shifts
- Rapid-eye-movement (the stage’s namesake)
REM sleep is also characterized by:
- Practically paralyzed muscles
- High brain activity.
We usually enter the first REM phase within 70 to 90 minutes of sleep.
Why Is REM Sleep Important?
We spend roughly one-fourth of each night in the REM sleep stage, and this is no coincidence. REM sleep stimulates the areas of our brains that are crucial for learning, i.e., making and retaining memories. REM sleep also enables and fosters:
- Cell reparation
- Bone and muscle growth (hormones that do this are secreted during REM)
- Creative brain activity
- Vivid dreams
- Memory consolidation
- Emotional processing
- Cognitive functioning
Studies show that the lack of REM sleep can lead to migraines, confusion, severely limited cognitive capacity, and other consequences.
REM sleep has a central role in healthy rest. Unfortunately, it is the most fragile sleep phase of all. Medication, alcohol, coffee, and sleep disorders all negatively affect REM sleep quality and quantity.
On the flip side, 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 throughout this stage.
What Affects the Duration of Sleep Stages?
It has now been confirmed that the distribution of NREM and REM sleep changes as we age.
Newborns spend anywhere from 16 to 18 hours asleep, and over 50% of that time is dominated by REM sleep. However, their sleep patterns soon begin to change.
At four months old, and later at eight months and 18-months old, babies experience poor sleep periodically during a two- to six-week period called sleep regression. During the first sleep regression, the stages of sleep shift so there’s more NREM sleep. The subsequent poor sleep episodes are due to milestones like crawling and standing up.
Starting from ages two to five and into adulthood, REM sleep takes about 25% of our overall sleep time. Nevertheless, the N1 and N2 phases slightly increase as we age. Therefore, we can predict the sleep cycle length by age.
While it’s true that the REM stage gets shorter over time, the difference is not that drastic, even in those over 60 years of age. Study results show that REM sleep decreases by as little as 0.6% every ten years from age 19 until 75. In older people over 75, a slight increase in the REM sleep stages was observed.
Psychological stress attacks all sleep phases, particularly our REM sleep. Although it usually reduces REM length, it can also make it longer. Either way, sleep quality is affected.
Meditation, CBT, vitamins, or a potent CBD oil can help reduce the effects of stress. Sleep medication is also an option, but it can negatively affect some stages of the sleep cycle.
It’s no surprise that the hormonal changes during pregnancy can influence sleep. During the first trimester, the pregnant person sleeps longer, with frequent daily naps, but rest is less efficient.
Both anatomical and biochemical changes in pregnancy seem to go against a good night’s sleep. Luckily, special aids like pregnancy pillows can offer some comfort.
Progesterone and estrogen (the predominant female sex hormones) both reduce the stages of REM sleep during sleep cycles. It’s not yet clear if this is the cause behind the more frequent nighttime awakenings in pregnancy’s second and third trimesters.
Our lifestyle and what we consume affect sleep duration and deprive us of specific sleep stages. The most frequently affected is the REM stage. Here are some common culprits for poor sleep:
Food and drinks:
- Drinking alcohol
- Caffeinated drinks
Medications such as:
- Diet pills
- Recreational drugs
This one is pretty self-explanatory. Tossing and turning due to high ambient temperature, disturbing noises, or uncomfortable bedding is the worst enemy of restorative sleep. The good news is, you can learn how to create a sleep-promoting environment.
This includes restricting noises, lights, and electronic devices. Nevertheless, it all starts by choosing the perfect mattress for your body type and preference.
Sleep Disorders vs. the Stages of Sleep
Any type of sleep disorder has an impact on the phases of sleep.
Insomnia is a sleep disorder that can cause both quantitative and qualitative effects. 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 sleep phases. Both too much REM sleep or a lack of it can negatively impact our health.
Another common sleep disorder that greatly affects every sleep stage is obstructive sleep apnea (OSA). Those with it experience fragmented sleep, with multiple N1 stages and shorter or no N3 and REM sleep.
One of the sleep disorders directly caused by a faulty sleep stage mechanism is narcolepsy. Sufferers fall into REM sleep instantly, instead of going through NREM stages first. Poor sleep hygiene also deprives us of the refreshing sleep stages or modifies their transition.
REM sleep behavior disorder (RBD) happens during REM sleep when people begin to act out on their dreams, usually nightmares. In other words, these are the screamers and shouters.
Sleepwalking is a sleep disorder when a person walks and acts awake but is deeply asleep. Sleepwalking has been linked to alpha waves appearing in the N3 stage, even though these are typically tied to other stages of sleep; the waves that should be showing up are delta waves.
Other parasomnias include nightmares, sleep paralysis (when the body is paralyzed in the wrong sleep stage, outside of REM), sleep-related eating disorders, sleep terrors, etc.
When we get a good night’s rest, both our mind and body function as intended. High-quality sleep has numerous health benefits, from boosting our concentration to improving our immune system.
It’s vital for the body and mind to go through all the sleep phases, as each is essential to our well-being. Knowing your sleep schedule can help you prevent sleep deprivation; understanding your sleep means waking up refreshed.
What are the 5 stages of sleep?
In terms of the brain’s electrical activity, we go through five stages during a sleep cycle. These are:
- Wake stage (Stage 0)—alpha and beta brain waves
- NREM Stage 1 (N1)—low-amplitude mixed-frequency brain waves
- NREM Stage 2 (N2)—theta waves
- NREM Stage 3 (N3)—delta waves
- REM Stage (Stage R)—mixed and alpha waves.
How many hours of deep sleep should I get?
Adults need at least one to two 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 that it’s assumed 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 process emotions and consolidate your memories. Dreams are most common at this stage. Therefore, you have to obtain enough REM sleep.
REM takes up about 20–25% of sleep for most adults. On the other hand, excessive amounts of REM sleep increase the likelihood of suffering from depression.
What stage of sleep makes you feel rested?
Stage 3 of 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. During this stage, your heart rate and breathing slow down to their lowest levels, your muscles are relaxed, and it can be hard to wake you up.
Is REM sleep deep or light?
REM sleep or rapid-eye-movement sleep is the most intense sleeping stage, but it’s not deep sleep.
Deep sleep is characterized by slow brain waves, heart rate, lower body temperature, and breathing frequency, and happens during the N3 stage of NREM sleep. N3 sleep is considered the most restorative sleep stage in terms of physical recovery.
During REM sleep, on the other hand, all the above-mentioned physiological parameters are high and unstable. However, this doesn’t mean that REM sleep is light sleep: Awakening from this phase means slowed cognition and irritability — a condition dubbed sleep inertia. Essentially, sleep inertia means being drowsy and unable to learn or concentrate.
Are there 4 or 5 stages of sleep?
There are five stages in a sleep cycle. However, we only sleep in four of those. The fifth stage is wakefulness. Out of the four stages when we actually sleep, three are allocated to NREM (deep) sleep, and one is the REM sleep stage (light sleep).