Benefits of NREM & REM in the sleep cycle: The science of sleep and memory

Share on facebook
Share on whatsapp
Share on twitter
Share on linkedin
A man reminising about a fond memory of past

Table of Contents

If you come to think of it, it’s quite awe-inspiring. We can easily conjure up a detailed vision of a moment or an event that transpired years, even decades ago. But there’s a strong chance that we don’t remember something as recent as what we had for breakfast two days ago. The answer to why this happens lies in sleep. And how each of our sleep cycle processes our memories in ways that continue to astonish us as we discover them.

So let’s dive right into understanding the mysterious state of unconsciousness that we move in and out of every night. The multiple stages of sleep, brain waves and memory storage sites, and how each stage of sleep processes different types of memories.

Sleep architecture

With all its marvelous mysteries, there is one thing that we know for sure about sleep – we don’t sleep in one cycle or block. On the contrary, our sleep has multiple stages and levels. Most importantly, it’s cyclical in nature. It’s like a labyrinth with many levels, the next deeper than the last. And together, these characteristics make sleep architecture.

What is a sleep cycle? Are all sleep cycles the same?

On average, we have four to five sleep cycles each night. Each sleep cycle lasts an average of 90 minutes or one and a half hours. Together, these cycles take around 7 to 8 hours to complete, which is the average healthy time one needs to sleep every night.

A man sleeping in 4 to 5 sleep cycles

Each 90-minute sleep cycle comprises 4 sleep stages – 3 non-rapid eye movement stages, plus one rapid-eye movement (REM) stage. For example, let’s say Steve sleeps a good 8 hours each night. It’s 10 pm, his bedtime. This is how he will drift into sleep – 

Stages of sleep: NREM Sleep vs REM Sleep

What happens during NREM sleep?

This stage is our gateway to sleep. We always start each of our sleep cycle with this stage.

Stage 1 or N1

In this stage, Steve will start transitioning from being awake to falling asleep. It’s a light sleep stage, meaning that it will be really easy to wake Steve up while he’s in this stage. But if he continues with no disturbance, Steve’s heartbeat and breathing will slow down and his muscles will start to relax.

Most importantly, his brain will start producing low amplitude theta waves (4 to 7 Hz).

Stage 2 or N2

Now Steve is entering the second level of sleep. This is the stage before deep sleep and has the largest percentage in total sleep.

N2 sleep is also pretty light. You can still wake Steve up easily. But let’s not do that and see what happens. 

Steve’s heartbeat and breathing will further slow down, his body temperature will drop.

For the first time, two distinct brain waves will start appearing in Steve’s brain – Sleep spindles and K-complexes. 

Note that sleep spindles have a deep relation with memory consolidation. We’ll get back to this point later.

Stage 3 or N3

Now Steve has reached the final stage of NREM sleep, which is the first deep sleep stage. It’s also called SWS or slow-wave sleep because the brainwaves become delta waves (which are the slowest brain waves). In this restorative stage, Steve’s body will strengthen its immune system, repair and regrow tissues.

If you try to wake Steve up from this stage, it’ll be quite difficult. And in case you do wake him up, he’ll feel disoriented for a while.

So if you ever wake up feeling groggy, you now know which sleep stage you might have woken up from.

REM stage of sleep

What is REM sleep?

This is the fourth sleep stage, and also the last stage of a 90-minute-long sleep cycle. As Steve enters this stage, his brain activity will shoot up as he starts dreaming. If you were to compare his brainwaves with the ones when he was wide awake, you wouldn’t be able to differentiate. In some areas of the brain, the activity is 30% higher than when a person is awake! Our brain is far from passive in the deepest stages of sleep. 

Brainwaves of a man measured while he's in REM sleep.

Although his brain is super active, most of Steve’s muscles will be paralyzed. You don’t want to act out those vivid dreams, do you? His heart rate and breathing will increase during this stage.

Do all these stages and sleep cycles seem complex? Wait till you find out how each sleep cycle has different REM and NREM proportions that vary throughout the night. 

Are all sleep cycles the same?

Varying proportions of NREM and REM 

What it essentially means is that every sleep cycle doesn’t have the same proportion or percentage of NREM and REM sleep.

A figure showing the different proportions of REM and NREM sleep in the sleep cycle.

The first half of our sleep has a higher proportion of NREM sleep, while the latter half has a higher percentage of REM sleep. 

For example, Steve will only experience around 10 minutes of dreams in the first sleep cycle. However, he will experience around 30 to 60 minutes of dreams in the last sleep cycle before he wakes up.

Memory storage: Geography of the brain

The hippocampus has limited storage space and the neocortex has unlimited space
Our brain has a short-term storage site called the Hippocampus and a long-term storage site called the Neocortex.

Just like a computer has a permanent memory (Read-only Memory) and temporary memory (Random access memory), the brain also has similar storage centers.

The hippocampus, located in the temporal lobe, acts as a temporary warehouse for all the information our brain is taking in. It’s like a USB stick with limited memory space. And you need to swipe it clean periodically to make space for new information. The neocortex, on the other hand, acts like a permanent warehouse of memory, with a lot of space. Once here, the information is a part of your long-term memory. 

Each night, your brain processes the day’s information. Imagine all this information stored in thousands of tiny packets in the USB stick of your brain – the hippocampus. Some of this is useful information, like the notes you took in a class, or an important client that your boss mentioned. The other, like maybe the fact that your colleague Amy brought in Greek salad for lunch, is not so relevant.

Now as you sleep, your brain stores some of these packets of information in the permanent memory (neocortex). While the others are erased to make space for new information for the next day. 

Brainwaves and memory- Courier service in brain city

It is through brain waves that the packets of memory travel to and from the different regions of the brain. Brainwaves are produced when masses of neurons communicate with each other through synchronized electrical pulses.

A brainwave transferring a memory packet from short-term storage to long-term storage in the brain
Brain waves transfer information in memory packets in our brain, acting like a courier service inside our brain city.

Think of your brain as a well-connected city. Then the brainwaves would be the courier service that delivers information back and forth to different areas of the city.

But not all brain waves have the same frequency or speed. Some are slow and some are fast, and both speeds serve unique functions. Let’s say that you want to send a packet of important information to the far-away neocortex or the permanent memory warehouse. For that, you’ll need delta brain waves, which are slow but have far wider reach.

Think of AM and FM waves. The courier service inside brain city works similarly. FM waves have high frequency or speed but carry information for only a shorter range of distances. The AM waves, on the other hand, are slow (remember delta waves in N3?) and have a farther reach. They can take packets of information to far off regions in the brain (like from the short-term memory warehouse to the long-term memory warehouse). 

High frequency FM waves versus low frequency AM waves

The battle stage for memory consolidation: NREM vs REM

Now that we know where it’s stored and how it travels in the brain, let’s talk about memory.

The ways in which sleep benefits memory is truly a marvel of our evolution. It helps us before learning by preparing our brains for new information and memories. It also helps in cementing those memories, consolidating them, so you don’t forget them. This consolidation of memory played a vital role in the survival of our ancestors. Consolidation of memory helped in giving us an evolutionary advantage. In the age of hunter-gatherer, we had to remember a lot of things. Like a threat, a location for food, water, and herbs, which routes were safer, which faces were friends and which were foes, etc.

But which sleep period – NREM or REM – confers greater memory saving benefits? The question set the stage for a battle between the two sleeps.

The winner was NREM. For memory that was fact-based and textbook-like, Non-REM sleep proved to be far more superior for retention in comparison with sleep rich in REM.

But then, why did we evolve to have REM sleep at all? Does it have any functions for memory? How does NREM process other types of memory?

Let’s find out.

Different types of memories in NREM

Fact-based textbook memories

One of the modern pioneers in the realm of sleep science, Matthew Walker, conducted an experiment. The purpose was to demonstrate how sleep transfers packets of fact-based information from short-term memory in the hippocampus to long-term memory in the neocortex.

After a good sleep, important facts transfer to long-term storage site in the brain

A group of people were given to memorize a set of facts. MRI scans were used to see from which region of the brain was the information being retrieved. The results were:

  • Before having slept, the participants’ brains fetched memories from the hippocampus (the temporary storage site)
  • After having slept a full night, the same information was being fetched from the neocortex (the permanent storage site)

NREM sleep helps in this transfer of information through slow delta waves that are produced in the N2 stage.

Motor memories

Memorizing a fact like the capital of France, or your daughter’s ballet recital date is different from motor memories. Motor memories are like learning how to ride a bike, play a piano or guitar, or drive a car. Non-REM sleep yet again does a stellar job of consolidating them in your brain as you learn motor skills. How quickly and efficiently you learn a motor skill directly depends on the amount of the stage 2 NREM sleep that you get. Especially in the last two hours of an 8-hour sleep. 

What’s so special about stage 2 NREM sleep though? Remember sleep spindles? They are essentially bursts of brainwave activity that happen in N2 sleep. And it’s because of the high number of these sleep spindles in the last 2 hours of sleep that memory boosts happen.

You know what’s more amazing? The fact that after you learn a motor skill, the number of sleep spindles increase in the motor cortex. This is the region of the brain responsible for planning, control, and execution of voluntary movements. The sleep spindles do not increase in other areas of the brain!

Learning how to play the piano

For example, let’s say that Steve decides to learn how to play the piano. He starts with an easy melody, staccato style, playing some single notes in rhythm. He practices for a few hours but always misses 2-3 in the melody. However, when he falls asleep, sleep spindles will especially increase in his motor cortex because he’s been practicing a motor skill. His brain waves will place special focus on the area that needs more help. 

Thus, upon waking, Steve will be able to play the melody without missing any notes. This will happen as the motor memory is now consolidated in his brain. 

After a good night's REM sleep, you can remember motor skills better

Fact-based textbook memories are transferred from short to long-term memory. However, brain waves shift the motor memories to those brain circuits that operate below the level of consciousness. The motor skills become automatic upon consistent practice and their consequent consolidation in N2 sleep. That’s why if you’ve once learned how to ride a bike, or drive a car, those actions come automatically to your body. You don’t have to think about how you’d ride a bike every time you start to paddle one.

Significant memories – remembering and forgetting 

Sleep is not at all generic when it comes to preservation of memory. Instead, it intelligently tags certain information as significant and stores it in the long-term memory. While other information in the short-term memory is soon forgotten to make space for new memories.

NREM sleep (because of its sleep spindles) helps in remembering and forgetting information. The more sleep spindles a person has, the greater is the efficiency with which the information in their brain is tagged. Important information is tagged for remembering and irrelevant information is tagged for forgetting. 

During this process of tagging, two regions of the brain are active. One, the hippocampus (temporary memory storage site). Two, the regions that program the decision of whether an information is ‘important’ or ‘irrelevant’.

So while preparing for an exam, let’s say that you were consciously marking certain information as important. Then you have a good night’s sleep with complete NREM cycles. Now your brain will automatically save the important information in the permanent memory, making it easier to remember and recall in the exam.

The brain storing the information that you mark as important.

REM Sleep

With the countless benefits of Non-REM sleep in memory consolidation, one may wonder about why we evolved to have REM sleep. Well, mother nature doesn’t make mistakes. REM sleep has played a pivotal role in our evolution as a species. Some of its functions are:

Overnight therapy by providing you an emotional resolution for difficult, even traumatic emotions

In one of his experiments, Walker tested the age-old saying ‘Time heals all wounds’. What he found was that it was actually the time spent in dream sleep that heals emotional wounds. 

Our painful emotions and memories become less painful after REM sleep
REM sleep helps us processing our emotions

Turns out, our brains have an anxiety-triggering chemical called noradrenaline. And this chemical is present in our brain all the time, except for a specific window of time. And guess when that time is? It’s during REM sleep! 

So basically, during a 24-hour period, it’s only during REM sleep that our brain is free of a chemical that triggers anxiety. However, certain parts of the brain are still active when we are in REM sleep and dreaming. These include various regions of our brain. Like the –

  • Visuospatial regions (because of which we see and navigate places in our dreams)
  • Hippocampus and regions around it that support our autobiographical memory
  • Regions that help us generate and process emotions (the technical terms would be the amygdala and the cingulate cortex).

To understand it better, let’s say that Steve went through an event that caused him a great deal of emotional pain. Each time Steve sees the event in his dream, he relives it in an environment that’s free of stress chemicals. This means that in REM sleep, you see and live the event free from the emotional pain that it caused in real life.

The memory of the event would still be there, as a part of Steve’s autobiographical knowledge. But over time, the painful emotions associated with the memory would fade.

In this way, REM sleep helps us remember events as information, while detaching the emotional pain and trauma associated with it. 

Helping us decode waking experiences by fine-tuning our brain’s ability to detect emotions

One of the basic prerequisites of being a functional human being is our ability to read expressions and emotions of faces with precision. Through facial expressions, we can communicate with other people, detect their intent, and behave accordingly. This is a huge evolutionary advantage of our species. It’s also something that we take for granted.

Is this ability to detect a person’s expressions and emotions innate to us? By any chance, can this ability be sharpened, and similarly, worsened? 

A woman misinterpreting someone's facial expressions because of lack of REM sleep.

If you deprive a person of REM sleep, their ability to decode facial expressions will become distorted. They won’t be able to tell the difference between a friendly smile and a sarcastic snide! Imagine the consequences of this in a time when languages weren’t that developed and people relied on gestures and expressions to guess someone’s intentions.

Problem-solving and creativity

We know that NREM sleep consolidates memories. Information travels to the long-term storage site of the brain through slow waves generated in NREM sleep. But it’s in REM sleep, when you’re dreaming, that a truly unique phenomenon happens. The current information is positioned with past information, experiences, and memories. This is done to draw insights and countless links. This sparks creativity and solves complex problems that a waking mind can’t or is unable to.

A popular phrase related to this is found across different cultures. Whenever there’s a problem that you can’t solve, people often advise you to ‘sleep on it’. While you are in the REM stage of your sleep cycle, your creative memory will be processed. The information that you need to solve the problem will move beyond logical connections of your awake mind. It will create shortcuts to new links between concepts that are differently related to the problem you’re trying to solve. It will draw connections and associations between all the relevant information that’s stored in the deepest cabinets of your brain.

Many scientists and artists have fascinating stories of stumbling upon a solution or a new inspiring idea after waking up from a dream.

Did you know?
The Beatles’ classic hits ‘Yesterday’ and ‘Let it be’ came to Paul McCartney in his sleep?

Thank sleep for your memories

A person with numerous memories stored in his brain.

Memory is one of our evolutionary gifts. We can cherish people, events, and moments for our entire lives. Through our ability to keep memories safe in our brains, we can do that long after they have passed. Today, we have discovered how sleep helps process, integrate, and consolidate memories every single night. Also, how important it is to sleep well.

Someday, you might find yourself fondly reminiscing about a day that passed years ago. Remember to thank that night’s sleep safekeeping that memory, and for moving it to your repository of permanent, long-term memories in your brain.

Subscribe to Our Content

Get Sleep Tips, Comics and Exclusive Offers Right Into Your Inbox.

Share the article

Share on facebook
Share on whatsapp
Share on twitter
Share on linkedin

Leave a Reply

Your email address will not be published. Required fields are marked *

Disclaimer: The information on our site is only meant as general information. It is NOT medical advice for any specific person or condition. If you have any medical questions and concerns, please contact your healthcare provider.

Join to get sleep tips and product updates right into your inbox.

479 Jessie St, San Francisco, CA 94103, USA

©2021 Cradlewise, Inc. | All Rights Reserved

Made with by Tech Savvy Parents

Your Cart is empty!

It looks like you haven't added any items to your cart yet.

Browse Products






One on One video call with the founder. See Cradlewise in action.



get $100 OFF

Register for a Live Demo & AMA every Saturday


One on One video call with the founder. See Cradlewise in action.



Copy the coupon code and use on the checkout page to get $200 off on the crib.



We'll bring the crib at a driveway/park close to you.



See Cradlewise in action. 1-on-1 video call with the founder.



See Cradlewise in action. 1-on-1 video call with the founder.






get $200 OFF

Thanks for signing up!

Know someone who wants to join the fight?

Share the website to let them know.

Share on facebook
Share on twitter
Share on email
Share on whatsapp

We are all in this together.

Join the fight against COVID-19.

Please bear with us. We will send you an update only when needed.

Join the waitlist.
Launching all over the US Jan 2021.

Signup to get first preference.

We hate spam as well. We will send you an update only when needed.

Signup to get sleep tips and product updates right into your inbox.

Thanks for signing up!

Know someone else who is expecting a baby soon?

Share the website and help them out

Share on facebook
Share on twitter
Share on email
Share on whatsapp



Copy the coupon code and use on the checkout page to get $100 off on the crib.