2. Sleep Stages Explained

TL;DR: You cycle through different stages of sleep: light sleep, deep sleep, and REM, with the last two being most important. Keep your room dark and between 15 – 19^oC, eat during the day, and most importantly, sleep while the sun is down and be awake and active while the sun is up.

“Our brainwave profile and our daily experience of the world are inseparable.” – Brainworks, one of the first neurofeedback clinics [1]

What Are Sleep Stages?

Sleep stages are distinct types of sleep that occur at different times and achieve different purposes in the body. There are two recognized main stages of sleep: Rapid Eye Movement (REM) and Non-REM. Non-REM sleep is further split into three stages: N1, N2, and N3, with higher numbers indicating deeper sleep. Sometimes the N3 stage is split into N3 and N4 sleep, but it will be referred to as just N3 sleep in this article.

When we fall asleep we first enter N1, then N2, then N3, and finally REM sleep. Going through a full cycle like this takes approximately 90 minutes. This cycle repeats 4 to 5 times per night [2].

Each stage is characterized by different brain frequencies. In our waking state, our brain frequencies are fastest, and it slows as we fall asleep and enter N1, then N2, then N3, and finally REM sleep where the brain frequency speeds up again. Stages N3 and N4 are also called “deep” sleep, while N1 and N2 are “light” sleep [3]. Deep sleep, light sleep, and REM are the three key stages to remember.

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Sleep is measured using an electroencephalogram (EEG), which records when neurons in the brain fire – see above. Different frequencies of neuronal firing are identified using an EEG, and then this information is shown graphically in a hypnogram, see below [5]. EEGs are used to record brain activity in general, not just for sleep, so what is learned about sleep stages helps us learn about the brain in general. Modern sleep studies measure brain waves (EEG), eye movement (EOG), and muscle activity (EMG) [6].

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Music is a helpful analogy to understand brain waves. Brain waves are a frequency much like musical notes, which are sound waves. Deep sleep, which is slow frequency, would sound more like a slow, powerful drumbeat, and high frequency REM would sound more like a high pitched flute. Some companies even use this principle outside of sleep to provide therapy to people by letting them hear their own brain waves – it’s called neurofeedback.

Why Do Sleep Stages Matter?

Sleep stages matter because each stage of sleep accomplishes different purposes. If certain sleep stages are not being reached properly, the recovery accomplished by that sleep stage will be hampered.

Each sleep stage is characterized by different brain frequencies, which are a measure of how fast neurons are firing in the brain. Neurons don’t just fire randomly though, they fire in timing with large groups of other neurons. If you zoom out and look at all frequencies in the brain at any given time, the firing of neurons is synchronized and organized together. This allows us to say that someone is in the alpha brain state or beta brain state, or a mix of the two etc. One type of wave usually dominates in the brain and can be identified.

It is still not clear what exactly happens in each sleep stage, just as it is not clear why we even sleep in the first place. However, it is clear that REM and deep sleep are critical, and it appears that deep sleep in particular is what makes us feel well rested, and is what is most important for people to improve on.

Here are the commonly found frequencies in the brain:

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1 Hz = 1 cycle per second, so if you are in the alpha state for example, the neurons in your brain are firing at about a rate of 10 times per second.

As you can see in the image, several different main frequencies occur in the brain, and they each have different functions. Understanding brain waves is key to understanding the different stages of sleep. Listed below is a description of each stage of sleep and which brain frequency it corresponds to. As you work down the list below, you move into slower and slower frequency brain states.

In Waking State (Beta waves and Gamma waves) you are fully conscious, and your muscles are alert and active. Most people are typically in this state during day time.

REM

REM (Alpha and Beta) is sometimes called paradoxical because EEGs show similar brainwaves to when one is awake even though they are asleep. It seems that the body goes to sleep, but the mind stays awake, which may be why dreams are most common during REM sleep. In fact, all of the skeletal muscles in the body except for those used for breathing and eye movement are paralyzed – possibly to stop from moving around during dreaming [6]. It typically consists of 20 – 25% of total sleep time. [9]

REM is characterized by rapid eye movement, relaxed muscles, and vivid dreaming. The alpha waves that begin to dominate here are also found when people are daydreaming or in light meditation [10].

The function of REM is still unknown [11], but here is what we do know:

Deprivation of REM reduces neurogenesis in the brain (creation of new brain nerve cells) [12].
In another study, REM sleep deprivation caused mice to appear to develop metabolic syndrome related changes [13].
Interestingly, even in people who have permanent damage to the REM-generating part of the brain seem to not have a loss of function.
One fact that may shed light on REM sleep is that whatever it does, babies need a lot of it. In fact, of their 16 hours of daily sleep, 50% of it is REM – more than double an adult.
Another piece of information that gives some clues is during REM, the body suspends central homeostasis and allows large variation of breathing, temperature regulation, and circulation. This doesn’t happen in any other sleep or waking phase. [14]
One last fact is that REM is unique to mammals and birds [15].

Light Sleep

N1 (Alpha waves, then theta waves) sleep is the lightest stage characterized by slow eye movement, and alert and active muscles. You can hear sounds, and feel things, but are not willing to respond. This stage typically lasts only 10 minutes, and comprises about 5% of total sleep time. [9]

As you slip deeper into sleep, you move into N2 (theta waves) sleep, where awareness of your environment fades and muscle activity reduces. N2 is the longest stage of sleep, comprising 45 – 50% of total sleep time. [9]

Deep Sleep

N3 (Delta) sleep, also called slow wave sleep or deep sleep, is perhaps the most important phase of sleep where deep recovery occurs. It is characterized by being unaware of the external environment – light, sounds etc., and breathing rate, heart rate, blood pressure, brain temperature and brain activity all slow down to their lowest levels.

Dreams are unlikely before this stage, and are possible in this stage, but most dreaming happens in REM. Sleepwalking and night terrors may happen in this stage, and memory consolidation and information processing occur. Waking up in this stage is difficult, and often results in grogginess, which is why some sleep apps try not to wake you in this deep stage of your sleep. N3 sleep should make up 15 – 20% of total sleep time [9].

During deep sleep, human growth hormone production begins [16]. In addition, norepinephrine, serotonin, and histamine receptors turn off, as shown in rat studies, which may allow these receptors to rest and resensitize [9].

A particularly important aspect of deep sleep is waste removal in the brain. The lymph system in the body performs many immune related functions, one of which is removal of waste products from metabolism. Recently, it was discovered that the lymph system extends into the brain, and is specifically called the glymphatic system there because it relies on glial cells in the brain. It is only really active during sleep and not used while awake. A study in 2019 found that the glymphatic system activity peaks during deep sleep. This seems to be because the glymphatic system operates most efficiently, at least in mice, when the brain is operating in the slow delta waves of deep sleep [17]. Furthermore, when waste products are not removed, it can predispose to neurodegenerative diseases such as Alzheimer’s, which is associated with accumulation of amyloid beta and tau proteins in the brain [16].

Another interesting factor in deep sleep is physical activity. One study in 2014 found that exercise one and a half hours before bedtime was associated with increased deep sleep and decreased REM – especially in those who self-reported that they had a hard workout. This indicates that deep sleep may play a key role in recovering from physical activity [19].

Naps

During a 20 to 60 minute nap, a person goes through cycles similarly to a 90 minute cycle of sleep. This leads some to believe that naps are more efficient than a regular night of sleep, but very few believe that just taking naps can replace sleep entirely. Occasionally, or even daily, it may help those who need to fit some extra sleep in their day with less time [9].

How Do I Optimize My Sleep Stages?

Follow the tips below for achieving uninterrupted and healthy sleep.

Circadian Rhythm Optimization

The first thing to work on when it comes to sleep stages is our circadian rhythm – as in getting up when the sun rises and going to sleep when it sets, or getting as close as you reasonably can. When the timing of our cells is correctly aligned, all sleep stages will be improved at their root causes. Refer to our article on circadian rhythm optimization.

REM and Deep Sleep

REM and deep sleep seem to be the most important sleep phases for recovery.

One impact of circadian rhythm on sleep stages is simply that the earlier you fall asleep, the more deep sleep you will likely get, and the less REM. Ideally, you want to balance the right ratio of deep sleep and REM sleep, so going to bed at a different time will affect these ratios and you can experiment with what feels good for you. Many people seem not to get enough deep sleep, so going to bed earlier may help many people. As they say, “Early to bed, early to rise makes a man healthy, wealthy, wise”

When it comes to REM, it seems it is particularly susceptible to drugs and medication, as well as temperature. Caffeine, alcohol, marijuana, opioid pain medication, benzodiazepine medications, antidepressants and lithium all suppress REM sleep [11]. In terms of temperature, ideally you want to sleep in a room that is 60 – 67 °F / 15 – 19 °C. Temperatures above 71 °F / 24 °C as well as below 53 °F / 12 °C are likely to impair sleep quality [16].

Sleep Consolidation

Insomniacs have difficulty falling asleep and staying asleep, which often disturbs sleep cycles. One counter-intuitive method for remedying this is to dramatically cut the amount of hours spent in bed, but only at first. The idea is that instead of spending hours tossing and turning, you instead deprive yourself of sleep temporarily, then sleep deeply out of necessity. This consolidates your sleep so that it is no longer interrupted and you can make it through full sleep cycles. This will train your body to associate bed time with actually sleeping rather than tossing and turning. After you adjust, you slowly return back to a normal amount of sleep. Read more on sleep consolidation [11].

Even for those who are not insomniacs, it seems to be better to get out of bed and wait until you are tired than to toss and turn even if it means getting less time in bed overall.

Sleep Stage Recording

Using devices that record your sleep allows you to take knowledge of sleep cycles much further. One device example is the Oura ring, which starts at $299 USD. It records the % time in each sleep stage, shows you a basic hypnograph, total sleep time, how long it takes you to fall asleep, and other measurements related to sleep and recovery. You can compare your sleep stage %s to the typical healthy %s listed above and see where you stand. Then, you can tweak your lifestyle and see how it affects different stages of sleep. Try out the Oura ring or another device that gives you a deeper look into your sleep.

Brainwave Entrainment

The idea of brainwave entrainment is that external electromagnetic fields propagating through the brain cause the brain to sync with the external frequency. For example, playing delta waves through ear buds will start to entrain your brain to enter delta wave state. Over time, this effect becomes more powerful as you repeat this and your brain learns to entrain itself.

A specific example of this is Pulsed Electromagnetic Field therapy (PEMF) devices. In one study, PEMF therapy helped 90% of insomniacs in the experimental group vs. 50% in the placebo group based on the following measurements: sleep latency, frequency of interruptions, sleepiness after rising, daytime sleepiness, difficulty with concentration, and daytime headaches [10]. Read more on PEMFs and sleep [19].

Another example that doesn’t cost anything are binaural beats, which may help you shift from beta waves (alert and awake) to alpha waves (relaxed and awake), which is like a stepping stone to sleep. There are many different frequencies of binaural beats, and you can play them through regular headphones as you fall asleep. This may help those who are wired, and have trouble getting relaxed before sleep.

Genetics

Genetics affects the distribution of different sleep stages. In fact, the profile, or wave that shows in a person’s EEG for REM and N-REM sleep is one of the most genetically determined characteristics identified to date [20].

Some other articles on UpRiver that we recommend for optimizing sleep include: nighttime routines and bedroom hacks. For an extra edge in your sleep arsenal, see our articles on sleep supplements and devices.

Summary

Each stage of sleep is characterized by a different brain frequency, and achieves a different purpose in the brain. As you fall asleep, you first enter light sleep (N1, and N2), then deep sleep (N3), then REM. Deep sleep seems to be the most important phase where a lot of physical recovery occurs as well as clearing of toxins from the brain. REM is also important and helps you to create new neurons, dream, and perhaps consolidate memories. The most important things a person can do to improve their REM and deep sleep stages is to sleep roughly in timing with the sun, to only be in bed when you are tired rather than tossing and turning, and to keep your room between 60 – 67 °F / 15 – 19 °C. If you want to go deeper into sleep improvement, try out a sleep tracker like the Oura ring, and try different methods of brain entrainment.