Congratulations! You’ve found your way to the ingredients list of sleep. Having a clear sense of the constituents of sleep is a good place to start in the commendable journey towards identifying what constitutes quality sleep. 

Unlike what scientists thought way back when, sleep isn’t just one long stint of coma-like unconsciousness. Like any good recipe, a restful night of sleep is composed of a few primary ingredients added together in a particular order and in the right ratio. 

The right amount of sleep is essential for your body to recover from the stresses of your daily activities and process new ideas. Sleep is comprised of subcomponents called stages, defined primarily by electrical activity in the brain. While you’re asleep, you cycle through these different sleep stages in a particular order. 

There are two main types of sleep, during which, you accomplish different outcomes for the body and mind. The two types include non-rapid eye movement (NREM) and rapid eye movement (REM). You can read about REM sleep in this article. 

NREM sleep is made up of three distinct stages. Each NREM stage is similar, but they serve different purposes. What is the purpose of NREM sleep in your life? How do you know that you’re getting enough NREM sleep? Let’s find out.

What is NREM sleep?

NREM sleep accounts for three of the four sleep stages in one sleep cycle. There are three distinct NREM stages and one REM stage per cycle. 

Each of these NREM sleep stages can vary in length from five to 60 minutes of your total 90-minute sleep cycle. The longest of these sleep stages being stage 2 of NREM, which comprises approximately 40-60% of total sleep time.  

You might have read elsewhere that there are four NREM stages and here we are saying that there are only three. As recently as 2008 the American Academy of Sleep Medicine decided that the difference between NREM stage three and stage four was inconsequential and started referring to them both as N3. You might also see it here and there as N3 (and N4), and in some cases, some still use N4 — apparently old habits die hard. 

NREM Stages

Each night you cycle through all of the sleep stages between five to six times over a period of seven to nine hours. For each complete cycle, you move from N1 to N2, N3, and finally to REM. 

Each stage of NREM sleep is slightly different, but there are some similar characteristics of these stages of sleep. 

In the later stages: your body relaxes, blood pressure drops, brain activity slows, and you fall into a deep sleep. You can dream in all stages of NREM, but the dreams won’t be as vivid and fantastical as they would in REM sleep. 

Here’s what happens in each of the stages.

NREM 1

This is a very light stage of sleep that starts your sleep cycle. So, you can consider this the transition to sleep, when you first dose off. This sleep is very light. 

Picture the guy on the bus whose head keeps on flopping over and then righting itself. You know who I’m talking about, if you haven’t been on a bus lately, this guy also flies and takes trains. He’s alert enough to pull his head to a normal upright position, but just asleep enough to drool on his shoulder. Sound familiar? 

During this type of sleep, your brain creates high amplitude theta waves. These are also known as slow brain waves, not to be confused with delta waves. These theta waves are at a frequency similar to that of daydreaming or open road freeway driving. 

Stage N1 sleep only lasts between five to 10 minutes. If you wake someone up during NREM 1, they might not think that they were actually asleep. 

NREM 2

This is the second stage of sleep. During this stage, your body temperature drops and your heart rate and breathing slow, deepen and become more regular. 

In stage N2 you shift to a deeper sleep and your muscles relax. This stage of sleep lasts for around 20 minutes. N2 is the sleep stage you spend most of your time, approximately half of the total sleep time.

Sleep spindles and K-complexes are also produced during this stage of sleep. Sleep spindles are a type of high-frequency burst of brain activity that occurs at the end of a slow wave, while K-complexes are high-amplitude peaks of activity that usually precede sleep spindles. 

These events can be measured on an EEG (electroencephalogram) and are thought to occur in order to shield the brain from external noises. The more frequent the sleep spindles, the more resilient a sleeper is to external noises that may awaken them. 

NREM 3

NREM 3 is the deepest level of sleep for your body. This stage is often known as the delta sleep stage, because your brain produces delta waves. These waves are the slowest frequency of brain waves, approximately ten times slower than brain activity while you are awake. 

Stage N3 sleep is often also called deep sleep, delta sleep or slow-wave sleep (SWS). In NREM 3, you wouldn’t be bothered as easily by noises or outside stimulus as you might in other sleep stages. 

In this stage of sleep your body continues to relax and your blood pressure drops. Your breathing rate drops as well. Your muscles aren’t paralyzed in this stage, however, which is why parasomnias such as sleepwalking and night terrors can occur in this stage. 

REM vs NREM sleep

During REM sleep your dreams become more fantastic and your brain becomes highly active, even more active than during wakefulness. This is the part of your sleep where your brain forms memories and addresses emotional stress. REM sleep is healing for your brain, where NREM sleep is seen as healing for the body.

Your body enters REM sleep about 90 minutes after you fall asleep. You need to go through all three stages of NREM sleep before you enter REM. 

During NREM sleep, your brain produces slower brain waves that are associated with a deeper sleep. So, it is easier to wake up from your REM sleep stage than N3 stage. It’s during the N3 stage that your brain is least active but most collaborative. These slow brain waves ripple across the brain, allowing disparate brain regions to communicate and transfer information. This in part is why this phase of sleep is so crucial for memory consolidation, where our brains store and strengthen new memories and skills. Without adequate deep sleep, we would never be able to learn new skills or transfer short-term memories into more resilient long-term ones.

NREM sleep is also the time that tissues in the body are repaired and your heart and vascular system get a break. Since your body is resting, it doesn’t require as much work from your organ systems. So, NREM sleep is seen as a time that your body gets to repair and restore itself. In REM sleep, the body builds bones and restores tissues thanks to a heavy secretion of human growth hormone, the great healer of the body. The immune system is likewise strengthened and restored. 

When you get older your body spends less time in NREM sleep, which is linked to more physical issues as you age. Sleepers under 30 years old might get around two hours of NREM sleep a night, while those over 65 years of age get around 30 minutes. 

How much NREM sleep is normal?

We get most of our NREM sleep earlier in the night, and then the balance shifts towards longer REM cycles towards the early morning. Everybody is different, but it is recommended that you get between seven and nine hours of sleep a night. In order to get your best deep sleep, aim to be in bed before early morning, possibly 11pm or earlier. If you wake up groggy, or you don’t have enough energy to get through your day, you might have a poor sleep schedule. 

In most cases, you cycle through all of the stages of sleep within 90-120 minutes of falling asleep. The seven to nine hours of sleep recommended per night by experts usually translates to five to six complete sleep cycles composed of roughly 75-80% NREM sleep. 

What is the deepest level of sleep in your sleep cycle?

Even though we spend much of our sleep cycle in NREM 2 each night, it’s actually NREM 3 where we get our deepest sleep. This sleep stage is also referred to as restorative sleep. Our blood pressure drops because our blood flows out to our tissues and muscles. 

We spend around 20% of the night in this sleep stage, and we sleep deeper during the first half of the night. Since this is the sleep stage for the deepest sleep, it can be difficult waking sleepers from N3. It also starts out as longer cycles and shortens over the night. 

How does your sleep cycle change with age?

The architecture of your sleep shifts throughout your life, which makes sense if you think about the life of a baby versus an adult. As a newborn, you don’t really have sleep waves, and you spend most of your time in active sleep, the baby version of REM sleep. This is important because babies need to wake up to feed and get their diapers changed. 

Toddlers have fully developed sleep patterns with sleep stages. Then, as an adult, sleep cycles become more challenging. It can take more time to fall asleep, and elderly sleepers frequently wake up in the middle of the night and achieve less deep sleep. 

Even though your body needs a similar amount of sleep your entire life, it can be hard to get this sleep without the assistance of sleep aids.  

The bigger picture: sleep cycle

While NREM sleep makes up approximately 75% of the time you spend sleeping, getting a better sense of sleep cycles can give you a clearer picture of what constitutes quality sleep. You can learn more about sleep cycles here. Also, check out this article on REM sleep.

For more on how to improve your sleep through sleep hygiene, read this. For details about traditional Chinese medicine looks at sleep, check this out.

Glossary

REM sleep: One of the two fundamental sleep states, along with NREM (Non-rapid eye movement) sleep. It is known as dream sleep, due to the presence of many dreams during this period. It is characterized by rapid eye movements, desynchronized brain waves, low muscle tone, and irregular breathing and heart rate compared to NREM sleep.

Sleep Spindles: Bursts of neural activity with a frequency of 11-16Hz during stage 2 NREM sleep. Thought to play a role in memory consolidation and shielding the brain from awakening from external sensory stimuli