Sleep is an active state – even though you don’t remember most of what happens during the night. You go through different stages of sleep, some of which are as different from each other as they are from being awake.
There are 5 stages of sleep that follow each other in a cyclical fashion: wake, relaxed wakefulness, light sleep, deep sleep and REM sleep. After reading this article, you’ll have a basic understanding of the sleep stages, what happens during them and how they affect you during the day.
Jump to section
- NREM vs REM Sleep
- Stages of Sleep
- What is REM Sleep
- What is Stage N1 Sleep
- What is Stage N2 Sleep
- What is Stage N3 Sleep
- Sleep Cycles
- Stages of Sleep Over Multiple Nights
- How Does Scoring Sleep Work
- How Do Sleep Trackers Define Sleep Stages?
- What Affects Sleep Stage Distribution
Stages of Being: When Two Become Three
– Are you asleep?
– I was.
Do you ever get the feeling that life is made up of two parts? On one hand, there are the hours you spend awake – being active, thinking, feeling, being you – and on the other the hours you spend asleep, your brain switched off and toes tucked away under the sheets.
It’s as if there were two stages of being, ON and OFF:
Since the advent of electric light, we’ve increasingly ignored and misunderstood sleep: It’s a necessary evil. It’s an inactive state. It’s the human stand-by mode. Why sleep when you could watch just one more episode on Netflix or go to Mars? However, with advances in science and technology, and a budding revolution in attitudes towards sleep, we may be ready to throw these ideas into the dustbin of history and embrace a non-binary view on human existence and see that there is no ON/OFF for us. Rather, there are three stages of being that are inextricably intertwined, each one affecting the others:
These three states of being are found in nearly all mammals and birds: wakefulness, Non-REM sleep and REM sleep. From a physiological viewpoint they are as different from each other as sleeping is from being wide awake. You just don’t notice it – mainly because you’re asleep.
– Are you dreaming?
– Fly me to the Moon, Elvis.
NREM vs REM Sleep
We know surprisingly little about sleep, despite the fact that it’s such a defining feature of our existence. For two thirds of the day we go about our business – live, love, eat, file the taxes – and then we put our phones on silent, turn off the lights and go into this weird mini-hibernation mode for 6–9 hours. For parts of the night, our body is paralyzed and our brain plays a remix of events and situations that were and could be (called dreams). When we wake up, we don’t remember much of what happened and we’re still learning about why.
There are some things we do know.
All mammals and birds sleep. Their sleep is divided into two states: REM (rapid eye movement) sleep and Non-REM sleep. This is the three-fold division of our existence: we’re awake, we’re sleeping and we’re dreaming.
REM sleep is a stage in which the brain is active but the body is paralyzed. The name comes from the fact that during REM sleep our eyes dart about underneath our eyelids. It’s during REM sleep that we have our most vivid dreams.
Non-REM sleep is a stage in which the brain is relatively inactive and our body is movable. Non-REM is further divided into stages based roughly on how deep the sleep is and how difficult it is to rouse someone from it.
When you drift into sleep, you first go into the lighter stages of Non-REM sleep. Through these stages you move into REM sleep. This fairly predictable progression from Non-REM sleep to REM sleep is called a sleep cycle. In these 90–110-minute sleep cycles you sleep throughout the night.
What feels like a complete blackout, or time-traveling, is in fact a fairly structured process that follows a predictable yet varied course.
REM sleep accounts for approximately 20–25% of total sleep time, whereas NREM sleep stages account for the rest. If you sleep for 8 hours, the percentages translate to roughly 1.5 to 2 hours of REM sleep per night.
Stages of Sleep
There are 5–6 stages of sleep, depending on which terminology you use. Fairly counterintuitively being awake can be counted as being one of them. According to the American Association for Sleep Medicine (AASM) classification, there are 5 stages of sleep:
- Stage W: Wakefulness
- Stage N1: Relaxed Wakefulness
- Stage N2: Light Sleep
- Stage N3: Deep Sleep, or Slow-Wave Sleep (SWS)
- Stage R: REM Sleep: Dreaming
The N in N1–N3 refers to Non-REM sleep. The AASM classification is the one used by Oura, except that we have combined N1 and N2 stages into one stage called Light Sleep.
An earlier classification further divided N3 sleep into two separate stages. According to this classification these are stages of sleep:
- Stage 1: Relaxed Wakefulness
- Stage 2: Light Sleep
- Stage 3: Slow-Wave Sleep (SWS)
- Stage 4: Slow-Wave Sleep (SWS)
- Stage 5: REM Sleep
Each sleep stage is characterized by changes in our physiology. However, the transitions between stages may not always be clear cut, partly because traditionally sleep has been assessed in 30-second epochs.
In the following sections we’ll go through each stage and take a look at what happens in your body during each stage, what you may experience and what are the functions of the different sleep stages.
What is REM Sleep
REM sleep is a sleep stage in which the brain is active but the body is paralyzed. Vivid dreams happen during REM sleep and your heart rate and respiration rate are increased. In terms of brain activity, REM sleep resembles wakefulness.
You tend to get more REM sleep later in the night. REM sleep may be completely absent from the first sleep cycles of the night. REM sleep is linked to the circadian rhythm of our body temperature: our core body temperature starts to drop off when we fall asleep and reaches a nadir in the early morning hours. If you go to bed much later than usual, you may skip the first cycles of sleep (including regenerative deep sleep stages) and even go straight to REM sleep.
According to studies on sleep deprivation, there seems to be a hierarchy when it comes to which sleep stages our bodies prioritize after loss of sleep: Deep sleep is recovered first, whereas REM sleep is recovered only after some nights of increased deep sleep.
REM sleep accounts for 20–25% of sleep in healthy, young adults. However, there are individual differences as well as night-to-night variation.
Physiology of REM Sleep
- Rapid Eye Movements
- Loss of muscle tone
- Low amplitude mixed frequency EEG
- Limited thermoregulation, shivering or sweating
- Elevated heart rate
- Elevated respiration rate
Experiences during REM sleep
- Vivid dreams
- Erections and blood flow to the genitals
Functions of REM Sleep
- Learning and problem solving
- Memory consolidation
- Mental health
What is Stage N1 Sleep
Stage N1 Sleep is a transition stage between wakefulness and the deeper stages of sleep. It is easy to wake up from this stage. During N1 you are aware of your surroundings but you become increasingly relaxed as you drift off to sleep. Illogical thoughts and sudden I-almost-fell-off-the-top-of-the-world twitches are normal during this stage of sleep.
During the first sleep cycle, you usually spend less than 10 minutes in Stage N1 sleep. It accounts for approximately 5% of total sleep time. If you sleep for 8 hours, 5% is 24 minutes in total.
Physiology of N1 Sleep
- Core body temperature drops at sleep onset
- Slow eye movements
- Lack of sleep spindles in the EEG
Experiences during N1 Sleep
- Hypnic jerks
- Illogical thoughts when drifting off to sleep
- Reduced awareness of surroundings
Functions of N1 Sleep
- Transition from wakefulness to the deeper stages of sleep
What Is Stage N2 Sleep
Stage N2 sleep is stage of light, regenerating sleep. It is still relatively easy to wake up from this stage of sleep.
As you fall asleep, the muscles in your upper airway relax and make your airways narrower. Have you ever wondered how you know someone has fallen asleep watching the TV? Even if you don’t see whether their eyes are open or not? It is often the sound of their breathing that gives them away.
N2 stage sleep makes up the majority of our sleep: 45–50% in healthy young adults. With 8 hours of sleep, that is 3.5–4 hours of N2 stage sleep per night.
Physiology of N2 Sleep
- Presence of EEG patterns: K complexes and sleep spindles
- Decreased heart rate
- Decreased blood pressure
- No eye movements
Experiences during N2 Sleep
- Fragmented dreams
- Brief arousals from sleep
Functions of N2 Sleep
- Rest and recuperation
What is Stage N3 Sleep
Stage N3 sleep, deep sleep or slow-wave sleep (SWS) is the most rejuvenating and restorative sleep stage. During deep sleep, the glymphatic system removes waste, such as neurotoxins and beta-amyloids, from the brain.
It’s very difficult to wake up from deep sleep. If you’ve ever had to take an early morning flight and woken up groggy and disoriented, it may be because you were in deep sleep. There are several ways to get more deep sleep, from establishing good sleep hygiene to taking hot showers before bed. However, keep in mind that all stages of sleep are important for health and good-quality sleep, not only deep sleep.
The amount of deep sleep declines with age. Older adults often report waking up more often during the night which is linked to the decline of deep sleep.
Physiology of N3 Sleep
- EEG: slow-wave brainwaves
- Decreased heart rate
- Decreased blood pressure
- Release of growth hormone
- No eye movements
Experiences during N3 Sleep
- Very difficult to wake up from deep sleep
- If awoken, disorientation and grogginess
Functions of N3 Sleep
- Cell repair and rejuvenation
- Replenishing glycogen
- Long-term memory
- Removal of waste from the brain: glymphatic system
Sleep Cycles: Putting it All Together
As we’ve seen, sleep is not a uniform block of nothingness. During the night, different stages follow each other in fairly predictable patterns, called sleep cycles. A normal cycle starts from N1, goes through N2 to N3 and then back up towards REM sleep.
The average length of a sleep cycle changes throughout the night. The first sleep cycle is usually shorter than following cycles, clocking in at 70–100 minutes. The average length of the sleep cycle later during the night is 90–110 minutes. If you sleep for 8 hours, you’ll usually go through five full cycles.
Deep sleep predominates the first cycles of the night. On the other hand, REM sleep predominates the sleep cycles of the last third of the night. This is why one way to ensure you get enough deep sleep is to stick to a regular bedtime: if you go to bed later in the night, you may skip the first sleep cycles of the night and go straight to the REM-predominated cycles.
Stages of Sleep Over Multiple Nights
There’s a phenomenon called the first-night effect that has been observed in sleep studies. In short, it means that whenever someone goes to a sleep lab, their first night’s sleep may be very different from a normal night’s sleep. It might be better, or might be worse, depending on the person. The main thing is that it’s different.
That’s why it’s important to look at sleep over the long term, over several days, weeks and months even. Just like one bad night of sleep won’t ruin the next day, neither will one night of good sleep necessarily fix problems that have been going on for longer.
By looking at multiple nights, you can start seeing how the stages of sleep affect each other. For example, deep sleep has been found to have a rebound effect: if you don’t get enough deep sleep one night, the amount of deep sleep the next night may be higher. The same goes for REM sleep.
And it’s not only one night affecting the next night. There’s also the day in between – the choices that you make during that day – that has a profound effect. The night is the mirror of the day, and wake and sleep are deeply connected.
How Does Scoring Sleep Work
Traditionally, the stages of sleep have been measured and scored in a sleep lab, based on three polysomnographic measures and their variation as you move from one stage of sleep to the next. They are: muscle tone, eye movements and brain activity.
Electromyogram (muscle tonus)
The electromyogram (EMG) measures your muscle tone, how relaxed and pliable your muscles are. By placing electrodes under your chin, a sleep technician can measure your muscle tone throughout the night. During REM sleep, your body becomes paralyzed and you lose muscle tone.
Electroculogram (eye movements)
Rapid eye movements are one of the tell-tale signs of REM sleep. Electrodes placed near the eyes can track how much and in which direction your eyes are moving during the night.
Electroencephalogram (brain activity)
Brain waves or electric activity in the brain are measured with an EEG device, with electrodes placed on your scalp. Slow-wave sleep, or deep sleep, gets its name from the slow delta waves that predominate the EEG chart. In the images you can also see the EEG patterns, such as sleep spindles and K-complexes that happen during N2 stage sleep.
How Do Sleep Trackers Define Sleep Stages?
Commercially available sleep trackers use a variety of sensors to detect different stages of sleep. The more advanced sleep trackers available for use at home, like the Oura ring, track sleep and sleep stages based on data collected from the 3D accelerometer (movement), heart rate sensor (heart rate, pulse wave amplitude) and body temperature sensor.
Here Today. Gone Tomorrow: What Affects Sleep Stage Distribution
There are many factors that affect your sleep stage distribution, including age, previous nights, circadian rhythms, temperature, drugs and sleep disorders.
Age: Infants, Kids, Young Adults, Old Age
Infants and kids have a very different sleep stage distribution from that of healthy young adults which is described in this article. Sleep is important in all stages of our life, especially in adolescence as we’re growing and our cognitive abilities are developing.
As we age, it seems that there is more variability between individuals.
Generally speaking, the older you are, the less deep sleep you tend to get. Especially older men get less deep sleep as they age. As we know, it’s harder to wake up from the deeper stages of sleep and as a result older people tend to wake up more often during the night. Their sleep becomes more fragmented and their sleep efficiency is lower. Older people also tend to take more naps during the day which affects their sleep during the night.
Older people also tend to go to bed earlier and wake up earlier because their circadian rhythm is pushed forward. Staying active and getting sun light at the right times helps set the circadian clock.
Sleep is a homeostatic process. How much sleep you need depends on how much good-quality sleep you have got recently. It also depends on brain metabolism. The more active your brains have been, the more sleep you need.
If you’ve been sleep deprived, you’ll often see a rebound in the amount of deep sleep on the first night after sleep loss.
In addition to the homeostatic sleep drive, also circadian rhythms affect your sleep. It’s easier to fall asleep at certain times of the day: in the early morning hours as well as in the afternoon.
If you go to bed later than usual, in the early morning, you may jump straight to REM sleep.
The temperature of the room you sleep in may affect your sleep stages. Extreme temperatures, both too cold and too hot, have a tendency to disrupt our sleep, especially REM sleep, because our body’s ability to thermoregulate is lower during REM sleep.
Some drugs suppress REM sleep, while others suppress deep sleep. A glass of wine may help you fall asleep faster but tends to lower the quality of sleep.
Sleep disorders, such as sleep apnea or narcolepsy, can have an effect on sleep stages. In sleep apnea, both the amount of REM and deep sleep may be suppressed.
Sleep Stages. So What?
Do you feel rested and energized during the day? If you do, you don’t have to worry about sleep stages. Your body is doing its job and stressing over percentages or minutes spent in each stage is counterproductive. Following basic sleep hygiene takes you 90% of the way towards better sleep.
Do you feel tired during the day? Do you usually fall asleep right as your cheek hits the pillow? Starting your journey towards better sleep and better health can start with tracking your sleep stages. If you see that you’re consistently not getting enough deep sleep, it could be a sign of something that should be checked by a sleep professional.
And finally, do you feel rested, energized and curious? If you’re the type of person who likes to keep tabs on their physiology or see how their body reacts to daytime activities, consider getting a sleep tracker that tracks the stages of sleep. You might unlock the keys to better health and productivity by seeing what has previously been unseeable.