The Stages of Sleep Cycles
There are four stages of sleep you go through: 1, 2, 3, and REM sleep. Your sleep is categorized in two parts: NREM (non-rapid eye movement) and REM (rapid eye movement). The first three stages are part of NREM. The fourth and final stage is REM.
When you fall asleep, you go through these stages cyclically. This means you enter stage 1, sleep until REM sleep is complete, then begin again with stage 1. It takes between 90 to 110 minutes for each sleep cycle (1 to REM), with each stage lasting between 5 to 15 minutes.
Interestingly, not all sleep cycles are made equal. Your first few sleep cycles have relatively short REM sleeps and long periods of deep sleep. As you sleep longer, your REM periods naturally gets longer and your deep sleep lessens.
What Are the Stages of Sleep?
There are four stages broken up into two categories: Non-REM (NREM) sleep and REM sleep. Non-REM consists of Stage 1, Stage 2, and Stage 3 while REM sleep is the 4th and final stage of a sleep cycle.
There are a lot of different biological changes that occur during your sleep which indicate the start and end of a new Stage. As you go through the stages of sleep in a cyclical manner, the duration of your sleep and each stage varies between periods of wakefulness.
Did you know? It was only recently scientists agreed to condense our sleep stage framework from 5 Stages to 4. They found that two of the stages (3 and 4) were categorically similar and can be put together as the “slow-wave sleep” (Stage 3), otherwise known as deep sleep. We’ll have more info on deep sleep later.
Wakefulness periods: This is during the time when your brain activity is at its highest. It is also defined by active muscle tone, which is the unconscious low level of contractions of your muscles.
Stage 1 Sleep
Stage 1 is the lightest stage of sleep. Within minutes and sometimes even within seconds, you nod off and your brain produces alpha and theta waves. You transition between sleep and wakeful states. If you were woken up, you might claim you were never asleep in the first place.
This stage is defined by slow eye movements and a drowsy state which can easily be disrupted. Your muscle tone is relaxing and your brain activities begin to slow. Some people might experience hypnic jerks. On some occasions, you might experience a sudden falling sensation while naturally drifting in and out of Stage 1 and wakefulness.
Hypnic jerk: Also known as a hypnagogic jerk, sleep twitch, and myoclonic jerk. This is an involuntary muscle twitch that occurs when you begin to fall asleep. In other words, you might make a sudden jerking motion with your legs or whole body during the first few moments of falling asleep.
Stage 1 of sleep is what people often get as “catnaps”. “Powernaps” are for Stage 2 sleep.
Stage 2 Sleep
Stage 2 is where things slow down even further. If you want to get a power nap, you’ll want to wake up after this stage. This light sleep is where your memory consolidates itself, shuffling short-term memory into long-term memory. Synaptic pruning occurs here, where your brain cleans its extra neuron and synaptic connections so it can become more efficient.
Your slow eye movements stop here and brain waves called sleep spindles and K-complexes begin to appear. It isn’t clear what these two “sleep structures” do and why they exist, but our best guess is it protects our brain from awakening from sleep. These two bursts of brain wave activities define Stage 2, along with the drop in your body temperature and the slowing of your heart rate.
Sleep spindles: These are sudden bursts of brain activity that specifically occur during Stage 2 of sleep. They’re called spindles because of how they appear when printed out on an EEG (electroencephalography) reading, which measures our brain activities. Scientists aren’t sure what purpose they serve. Aside from protecting our brain from waking up, we think they might be associated with refreshing our ability to learn.
K complexes: These are also sudden bursts of brain activities that occur during Stage 2 of sleep. Scientists hypothesize they might also serve as brain protective measures from awakening from sleep.
Stage 3 Sleep
Stage 3 is known as deep sleep or slow-wave sleep.This is the deepest part of your sleep where your brain produces brain waves known as delta waves. During this time, it’s hard for you to wake up because of all the goodie delta waves bringing you into an even deeper and more restorative stage of sleep. Your body works to repair muscles and tissues damaged during the previous day, stimulate growth and development so you can continue to grow, boost up your immune function to prepare for a new day, and begins building up energy for when you wake up again.
We’re not too sure what else happens during this time as research hasn’t been able to indicate clearly. One theory is the brain is refreshing itself in some way for a new day of learning. Stage 3 deep sleep also reduces your sleep drive. More on this later.
Parasomnia occurs often in Stage 3 during this deepest stage of sleep.
Parasomnia: Undesirable behaviors or events that occur during sleep. Parasomnias are currently understood as clinical phenomena that arise as brain transitions between REM sleep, non-REM sleep, and wakefulness. Some examples include sleepwalking, sleep talking, sleep paralysis, irregular heart rhythms, and night terrors. Nighttime kicking is also on the list, too!
REM sleep is the final stage of sleep known for rapid eye movement and dreaming. Most of your vivid dreams happen in this stage. They seem to have actually happened, but we know too well this isn’t true.
During this time, scientists believe your learning and memory functions are improved, as your brain continues its consolidation and processes information that occured the day before, storing them into long-term memory for later access.
Your eyes will begin to move side to side as your brain becomes more active than Stage 2 and 3 sleep. Paradoxically, your muscles become more relaxed but voluntary muscles become immobilized. You are, however, easier to wake up during this period than Stage 3’s deep sleep. Heart rate and blood pressure increases, and breathing becomes fast, irregular and shallow.
It’s also important to note the first few REM periods tend to be short, but the later REM periods become longer with each completed sleep cycle.
Did you know? You can feel groggy and seem like you didn’t have enough sleep if you were woken up during REM. We call this sleep inertia, where you experience heightened drowsiness for sometimes several minutes to as long as several hours!
What is a Sleep Cycle?
A sleep cycle is the complete progression of the stages of sleep, from Stage 1 to REM sleep. The sleep cycle begins with Stage 1 of sleep where you bounce between wakefulness and sleep. As you continue sleeping, you enter REM before returning to Stage 1.
The longer you sleep during the night, the more your REM sleep increases and the more benefit you get from this stage of sleep. Deep sleep also decreases accordingly to accommodate longer REM sleep periods. This seems to mean your body first finishes its physical repairs and maintenance work before spending more time on learning and memory functions.
How long is a sleep cycle? The first sleep cycle is around 90 minutes. After that, sleep cycles average between 100 to 120 minutes. On average, a person might get between 4 to 6 sleep cycles each night.
What is Deep Sleep?
Deep sleep occurs during the slow-wave sleep, or Stage 3 of NREM sleep. During this time, your brain waves begin to slow in speed and have large amplitude. These brain waves, known as delta waves, signify Stage 3 and offers the most restorative sleep experience. You also become difficult to wake up, and least likely to be affected by external stimuli.
Your body begins repairing and fixing itself during this deep sleep period. Human growth hormones are also released, as your body fixes damaged muscles and tissues. Your immune system also restores and improves itself.
We aren’t too sure what else happens during deep sleep despite having so many sleep-monitoring tools at our disposal. One guess is that the brain might be refreshing itself
It’s worth noting that deep sleep also reduces your sleep drive. This means the more deep sleep you get, the less sleep you need and the more likely you are to wake up. This is the reason why short naps during the day doesn’t dramatically affect your ability to fall asleep at night. If you end up taking a long enough nap and enter deep sleep, it might be difficult for you to fall asleep and stay asleep for the night.
Sleep drive: Your sleep drive is your need for sleep. Your body has a delicate internal system that balances wakefulness and sleep.The longer you are awake, the greater your need for sleep becomes. As this need accumulates, it creates a biological urge (the drive) to sleep.
When Does REM Sleep Occur?
Your first REM sleep occurs approximately 90 minutes after you sleep. For the average person, REM will last approximately 10 minutes. As you cycle through new REM periods, they get longer. The last REM sleep you have for the night can last as long as 1 hour. The average person will typically experience 3 to 5 REM sleep periods during the night.
During every REM sleep, you can have intense dreams that feel real. This is because your brain is much more active than the previous deep sleep stage. Your breathing becomes fast, irregular and shallow, your heart rate and blood pressure increases, and your voluntary muscles such as your arms and legs become temporarily paralyzed, preventing you from acting out your dream.
For babies, 50% of their sleep is in REM stage. On the other hand, 20% of sleep for adults is spent in REM sleep. As you age, you spend less of your sleep time in REM sleep.
In What Stage of Sleep Do Dreams Occur?
Dreams occur during the fourth and final stage of your sleep: REM sleep. REM sleep, short for rapid eye movement, has several distinct characterizations.
Your voluntary muscles become temporarily immobilized to prevent you from harming yourself and your breathing becomes fast, irregular, and shallow. Your heart rate and blood pressure increases almost to the same levels as when you are awake. Your brain wave activities also reach a level of activity that is close to your wakeful state (but not quite there).
Brain Waves During REM and Non-REM Sleep
There’s a mnemonic for associating EEG patterns with each sleep stage: BATS Drink Blood (B.A.T.S.D.B.)
- Beta waves – awake and alert
- Alpha waves – awake and relaxed, tired, and day-dreaming
- Theta waves – Stage 1 sleep
- Sleep spindles and K-complexes – Stage 2 sleep
- Delta waves – Stage 3 slow-wave sleep
- Beta waves – REM sleep, but not exactly beta waves but they look similar to them as high frequency, low amplitude waves
BATS Drink Blood is a helpful mnemonic for understanding and reading EEG patterns. Although it’s no substitute for having a sleep specialist interpret your EEG charts for you, it can help you read along and ask questions as well.
- Brain Basics: Understanding Sleep by the National Institute of Neurological Disorders and Stroke
- At A Glance: Healthy Sleep by National Heart, Lung, and Blood Institute
- What Is Deep Sleep and Why Is It Important? by Healthline