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
So, you’re wondering about sleep stages? Sleep researchers divide sleep into four different stages (five if you count being awake as a stage). Each stage comes with its own unique patterns of brain activity and benefits. We don’t fully understand all of the mysteries of each stage, but what we can tell you is – sleep stages are pretty cool. Let’s take a look at how they were discovered and what the heck they all mean!
The history of measuring sleep
Until around 1930, it was widely believed that our brains turned off during sleep. This couldn’t be further from the truth. In 1929, the invention of electroencephalograms (EEGs) allowed scientists to record brain activity during sleep. Sleep studies used EEGs and instruments that measured eye movement and muscle activity to define two main types of sleep. These are now known as rapid-eye-movement (REM) sleep and non-rapid-eye-movement (NREM) sleep. Within these types of sleep there are four sleep stages.
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!
There are four sleep stages in total. During a sleep cycle, your body spends time in each of the stages. On an average night, you have around three to four sleep cycles.
The four sleep stages are defined primarily by brain wave activity and eye movement. There are three stages of NREM sleep which then transition into REM sleep. In order to understand sleep stages, we need to introduce you to the types of brain waves that are associated with each sleep stage. Let’s take a look…
Brain waves during sleep
Here are five types of brain waves ranging from slowest to fastest:
Delta Waves: These occur during the deepest stages of dreamless sleep, and certain types of deep meditation and yoga. At the point when delta waves are present, you will not be distracted by your external environment. Delta waves are associated with restoration and healing. They tend to steadily decrease across our lifespan.
Theta waves: These brain waves occur during dream sleep and along with delta waves during meditation. They are also seen when we are daydreaming. They are associated with learning, memory, spatial navigation, and emotions.
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.
Beta Waves: These fast-acting brain waves are found during wakefulness and are most dominant when we are alert, attentive, and engaged in an activity. They indicate brain arousal and a mind that is engaged, which makes it so interesting that they’re found during dream sleep.
Gamma Waves: These are the fastest brain waves and are found during wakefulness. They are involved in our ability to sustain attention and perceive the world. Altered gamma waves have been found to be associated with mood disorders such as major depression and bipolar disorder. Techniques such as meditation and mindfulness can help to synchronize gamma waves, which are associated with healthy brain functioning.
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 is the lightest stage of sleep and only lasts around 5-10 minutes. It begins right after you fall asleep, so it’s pretty easy to be woken up from stage 1. This stage is characterized by:
Alpha and theta waves
Regular muscle tone
Slow, rolling eye movements
As you continue sleeping, your brain moves into stage 2, which lasts about 30 to 60 minutes. During this stage, your muscles become more relaxed and brain activity moves into slow-waves. It becomes difficult to be woken up from this stage of sleep and it’s characterized by:
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 where your body becomes fully relaxed. This stage lasts between 20 to 40 minutes. It gets harder to wake somebody up once they enter stage 3 sleep. If you are awoken from stage 3 you tend to feel fairly foggy. This stage is very important for muscle repair, tissue regrowth, and strengthening the immune system. It’s 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)! This stage is important for forming memories, stimulating your nervous system, and balancing chemicals in your brain. The first period of REM sleep lasts 10 minutes but they become longer as they recur. The last period of REM sleep can last up to an hour. It’s characterized by:
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.