THE BOTTOM LINE:
Researchers use physiological responses to measure sleep stages
Four sleep stages make up the different levels of your sleep cycle
There are unique health benefits associated with each sleep stage
You wouldn’t be alone if you admitted that you don’t put very much thought into what happens after you’re asleep. It used to be a common belief that our brains turned off while we slept. As it turns out, your brain stays very active throughout the night. As you slumber your brain and body cycle through different sleep stages that each have specific patterns of brain activity and unique benefits. Cool right? Let’s take a look!
The history of measuring sleep
Until around 1930, the idea that your brain “turned off” while you slept was widely popular. However, it was completely wrong. Researchers were able to start studying sleep (specifically, they began recording brain activity during sleep) with the invention of the electroencephalogram (EEG) in 1929. What they found in their sleep studies was the beginning of understanding what is happening while you sleep and why it is important.
We now know that there are two main types of sleep, known as rapid-eye-movement (REM) sleep and non-rapid-eye-movement (NREM) sleep. Within these types of sleep there are four sleep stages with their own unique characteristics and benefits.
Before we explore the sleep stages, let’s look at how they are measured...
How are sleep stages measured?
During sleep studies, scientists rely on a system of assessments called polysomnography (PGS) that captures metrics such as your brain activity, muscle tone, and eye movements during sleep.
Measuring brain activity
Your brain activity during sleep can be measured using EEG, which is considered the gold standard for measuring sleep.
How does EEG work? Your brain has billions of nerve cells that produce small electrical signals as they communicate. These signals form patterns called brain waves. During EEG measurements, electrodes are attached to different areas of your face and scalp (typically using an EEG cap over your head) to measure the patterns of electrical activity in your brain.
Measuring muscle tone
Your muscle tone has been found to differ between wakefulness and sleep, and can also be helpful in diagnosing some sleep disorders. Muscle tone is measured using electromyography (EMG) which captures your muscle’s response or electrical activity.
How does EMG work? Typically electrodes (between 2 and 4) will be placed on the chin, above and below the jawline, which will measure the electrical activity that occurs when your muscles move.
How does EMG measure my sleep? During NREM: Muscle movements vary during sleep stages and even in between sleep stages. For instance, muscle jerks (called sleep myoclonus) occur just before you enter deep sleep.
During REM: Your muscles generally don’t move during this stage. Research has found that powerful brain chemical systems work together to paralyze your muscles during REM. Otherwise, we’d probably be acting out our dreams.
Measuring eye movements
Your eye movements can be measured using Electrooculography (EOG). Research has found EOG to be an accurate measure of sleep stages on its own, and may even be the easiest to use!
How does EOG work? EOGs measure your corneo-retinal activity (which is just a fancier word for eye movements) by attaching pairs of electrodes around your eyes (sometimes with a simple eye mask making it possible to do this yourself from home). Similar to EEG, these electrodes will pick up even the tiniest movements your eyes make.
How does EOG measure my sleep? Rapid eye movements (REM): We’ve told you about REM sleep, so you already know that when you’re entering that stage of sleep your eyes begin to rapidly move from side to side. You can measure them by staring at a person while they sleep, but a much more accurate (and gentler) way to measure is with EOG.
Slow eye movements (SEM): In contrast to REM, there are also lesser-known SEM that can be indicative of transitioning to stage 1 sleep. EOGs record these movements as slower “eye-rolling” movements.
Now that you understand how sleep stages are measured, let's talk about them!
As we mentioned before, there are four sleep stages that you will transition through in a night. Each sleep stage has unique characteristics, which include brain wave activity, eye movements, and muscle tone. Additionally, there are unique benefits associated with each sleep stage.
The first three of your sleep stages are considered non-REM sleep, with the fourth and last being REM sleep. Transitioning through all four of your sleep stages will complete one full sleep cycle, and you will typically complete a full sleep cycle three to four times a night.
To start, let’s take a look at the brain wave activity that occurs during each one of your sleep stages:
Brain waves during sleep
Here are the five types of brain waves associated with sleep, ranging from fastest to slowest:
- Gamma Waves: You’ll find these fast brain waves during wakefulness, involved with how we perceive our world and encourage attention and working memory. Research has found that mood disorders, such a major depression or bipolar disorder, may be related to altered gamma waves. Techniques such as meditation and mindfulness can help to synchronize gamma waves, which can improve brain functioning.
Beta Waves: Another brain wave found during wakefulness, typically when you are the most alert and attentive. Interestingly, they’re also found during dream sleep, providing some evidence that the mind is aroused and engaged during dreams.
Alpha Waves: These brain waves can be found during wakefulness and during the lightest stage of sleep. Sometimes these brain waves are referred to as a “frequency bridge” between our conscious wakefulness and subconscious sleep. When awake, alpha waves indicate a relaxed and calm state of mind and continue to be present as your muscles relax and you enter light sleep. You can increase the power of your alpha waves through techniques such as mindfulness meditation.
- Theta Waves: These brain waves can be found during dream sleep as well as when you’re daydreaming. Additionally, they can be found along with delta waves when engaging in meditation techniques. Moreover, theta waves are involved with learning, memory, spatial navigation, and emotions.
- Delta Waves: When delta waves become present, you know that you’ve reached the deepest stage of sleep. During this time, it’s very unlikely for you to be awakened or disrupted by environmental factors. These waves are important because they’re associated with healing and restoration.
THE STAGES OF SLEEP
Now that you understand what defines a sleep stage, let’s take a closer look at the sleep stages themselves. Stages 1 to 3 are NREM sleep while stage 4 is REM.
This stage begins as soon as you fall asleep and only lasts around 5-10 minutes. It’s typically easy to be aroused from sleep during this time since it is the lightest stage. Moreover, it is characterized by:
Alpha and theta waves
Regular muscle tone
Slow, rolling eye movements
As your sleep cycle continues, your brain will transition to the next stage of sleep. During this stage, which lasts between 30 and 60 minutes, your brain activity moves into slow waves, your muscles further relax, and it becomes more difficult to be woken up. Other characterizations include:
Theta and then delta waves
A drop in body temperature
Slower heart rate and breathing
No eye movements
What are sleep spindles?
These are bursts of very fast brain activity that briefly and randomly occur during stage 2 sleep. Research has provided evidence that sleep spindles play an important role in strengthening memories.
You fall into a deep sleep when your body becomes fully relaxed, and you’ll stay in this stage for 20 to 40 minutes. It will be harder to wake you up once you enter stage 3 sleep, and if you are awoken during this time you tend to feel fairly groggy. The deepest stage of sleep is characterized by:
Delta waves increase
Heart rate and breathing will slow down further but become a regular pace
Blood pressure drops
No eye movements occur
Muscles relax and are unlikely to show movement
REM sleep is Stage 4 in your sleep cycle and is considered the “active” part of sleep where your brain activity is the closest to wakefulness. REM sleep is best known for the stage of sleep where dreams occur (although dreams can occur at any stage of sleep)! The first period of REM sleep will last for approximately 10 minutes, but they become longer as they recur. The last period of REM sleep in your sleep cycle can be as long as one hour! Other characteristics include:
Eyelids begin to flutter
Breathing becomes irregular
REM atonia: Muscles become paralyzed (so you can’t act out dreams)
Gamma, theta brainwaves, and other brainwaves
Brain energy use (oxygen and glucose) similar to wakefulness
The sequence of sleep stages
It would make sense to think that sleep stages go through the sequence of stage 1, 2, 3, 4, and repeat. You may be interested to learn that this is not the case. Sleep does begin with stage 1, then stage 2, then stage 3, but before your first cycle of REM (stage 4) occurs, your body first goes back into stage 2 sleep.
So the sequence is really stage 1, 2, 3, 2, 4?
In the beginning! After your first cycle of REM sleep, you will begin the dive back into deep sleep and then start the cycle over again. As the night goes on, your body will spend less and less time in the deepest stages of sleep and more time in REM sleep. You won’t return back to stage 1 sleep until you begin to wake up (whether that be due to sleep disturbances or because it’s morning). Just like a rollercoaster ride, your sleep stages take you on a ride up and down the levels of consciousness.
Now you know when the different sleep stages occur, but why do they occur?
Unique benefits associated with sleep stages
The reasons why we transition through different sleep stages is not well understood but researchers do know some really cool benefits that each type of sleep provides:
Light sleep benefits
- Your body processes memories and emotions
- Metabolism regulates itself
Deep sleep benefits
- Muscle repair
- Tissue regrowth
- Strengthening of your immune system
- Your human growth hormone (HGH) is produced and released
(95% of your HGH is produced during this sleep stage)
REM sleep benefits
- Resets norepinephrine, making the amygdala less sensitive to stimuli and less likely to overreact to something fearful.
- Memory reconsolidation: You can thank your REM sleep for the reason your short-term memories become long-term memories!
- More REM sleep can heighten your problem-solving skills!
- The logic area of the brain is suppressed but creative thinking runs wild, this allows reorganization of thoughts, emotions, and can improve higher-order thinking. It also explains why dreams are so weird.