If you come to think of it, it’s pretty 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 it processes our memories.

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. 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?

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 seven to eight hours to complete, which is the average healthy time one needs to sleep every night.

Each 90-minute sleep cycle comprises three non-rapid eye movements (NREM) and one rapid-eye-movement (REM) stage.

For example, let’s say Steve sleeps a good eight hours each night. It’s 10 pm, his bedtime. He will drift into sleep like this: 

What is NREM sleep?

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

Stage one or N1

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

Stage two or N2

Now Steve is entering the second level of sleep—this is the stage before deep sleep and has the most significant percentage in total sleep.

N2 sleep is also pretty light. Steve’s heartbeat and breathing will further slow down; his body temperature will drop in this stage.
Two distinct brain waves will start appearing in Steve’s brain for the first time—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 three 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 (the slowest brain waves). In this vital stage, Steve’s body will strengthen its immune system, repair and regrow tissues.

It’ll be pretty difficult if you try to wake Steve up from this stage. 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.

What is REM sleep?

The REM stage is the fourth sleep stage and 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. 

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?

No. Each sleep cycle doesn’t have the same proportion or percentage of NREM and REM sleep.

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 about 30 to 60 minutes of dreams in the last sleep cycle before he wakes up.

Where are our memories stored?

Just like a computer has a permanent memory (Read-only Memory) and temporary memory (Random access memory), the brain also has similar storage centers. Our brain has a short-term storage site called the hippocampus and a long-term storage site called the neocortex.

The hippocampus acts as a temporary warehouse for all the information our brain takes 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 data 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 valuable information, like the notes you took in a class or an important client that your boss mentioned. The other information is irrelevant.

Now, as you sleep, your brain stores some of these packets of information in the permanent memory (neocortex). At the same time, your brain erases the other packets of information to make space for new information for the next day. 

Memory travels through brainwaves

Through brain waves, the packets of memory travel to and from the different regions of the brain. Masses of neurons produce brainwaves when they communicate through synchronized electrical signals.

Brain waves transfer information in memory packets in our brain, acting as a courier service.

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 city areas.

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 a 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 shorter distances. On the other hand, the AM waves 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). 

Does sleep help improve memory?

How sleep benefits memory is truly a marvel of our evolution. Before learning, it helps us prepare our brains for new information and memories. It also helps cement 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-gatherers, we had to remember many 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 more extraordinary memory-saving benefits? The question set the stage for a battle between the two sleep stages.

The winner was NREM. NREM sleep proved far superior for retention for a fact-based and textbook-like memory than 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?

Memories in NREM sleep

Fact-based textbook memories

One of the modern pioneers in 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.

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

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

Thus, NREM sleep helps transfer information through slow delta waves 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 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 stage two NREM sleep you get, especially in the last two hours of an 8-hour sleep. 

What’s so unique about stage two 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 two hours of sleep, memory boosts occur.

Do you know what’s more impressive? After you learn a motor skill, the number of sleep spindles increases in the motor cortex. This brain region is responsible for planning, controlling, and executing voluntary movements. The sleep spindles do not increase in other areas of the brain!

For example: Learning how to play the piano

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. Steve practices for a few hours but always misses some notes in the song. However, when he falls asleep, sleep spindles will significantly increase in his motor cortex because he’s been practicing a motor skill. His brain waves will focus on the area that needs more help. 

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

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 consciousness. The motor skills become automatic upon consistent practice and their consequent consolidation in N2 sleep. That’s why riding a bike or driving a car comes automatically to your body once you’ve learned it. You don’t have to think about how you’d ride a bike every time you start to paddle one.

Remembering and forgetting significant memories

Sleep is not at all generic when it comes to preserving memory. Instead, it intelligently tags certain information as significant and stores it in long-term memory. In comparison, the brain forgets the other information in the short-term memory to make space for new memories.

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

During this process of tagging, two regions of the brain are active. One, the Hippocampus (temporary memory storage site). Two, the areas that decide whether a piece of information is ‘important’ or ‘irrelevant.’

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

Memories in REM Sleep

With the countless benefits of Non-REM sleep in memory consolidation, one may wonder 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.’ He found that the time spent in dream sleep heals emotional wounds. 

REM sleep helps us process our emotions.

It turns out that 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 24 hours, it’s only during REM sleep that our brain is free of a chemical that triggers anxiety. However, certain brain parts are still active in REM sleep and dreaming. These include various regions of our brain. Like the:

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

For example, let’s say that Steve went through an event that caused him much emotional pain. Each time Steve sees the event in his dream, he relives it in an environment free of stress chemicals, which means that in REM sleep, you see and live the event free from the emotional pain 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 essential prerequisites of being a functional human is our ability to read the expressions and emotions of faces with precision. We can communicate with other people, detect their intent, and behave accordingly through facial expressions. 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? By any chance, can this ability be sharpened, and similarly, worsened? 

Research states that if you deprive a person of REM sleep, their ability to decode facial expressions will become distorted. They won’t be able to distinguish 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 knowledge, experiences, and memories. The brain does this to draw insights and countless links — sparking creativity and solving complex problems that a waking mind can’t or is unable to.

Different cultures across the world have a famous saying related to this. Whenever there’s a problem that you can’t solve, people often advise you to ‘sleep on it.’ Your brain will process your creative memory while you are in the REM stage of your sleep cycle. The information you need to solve the problem will move beyond the logical connections of your awake mind. It will create shortcuts to new links between concepts differently related to the problem you’re trying to solve. It will draw connections and associations between all the relevant information 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.

The bottom line 

Memory is one of our evolutionary gifts. We can cherish people, events, and moments for our entire lives. 

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 for safekeeping that memory and for moving it to your repository of permanent, long-term memories in your brain.

Sources:

1. Stages of sleep. Healthline. 2021. “Everything to Know About the Stages of Sleep.”
2. Comparison of brainwaves with AM and FM waves. Penguin Random House. 2017. “Why We Sleep.”
3. NREM sleep and fact-based memory. ResearchGate. 2008. “Sleep-Dependent Memory Processing.”
4. Emotional therapy in REM Sleep. NCBI. 2010. “Overnight Therapy? The Role of Sleep in Emotional Brain Processing.”
5. REM sleep and ability to recognize facial expressions. NCBI. 2010. “Sleep Deprivation Impairs the Accurate Recognition of Human Emotions.”
6. Creativity in REM Sleep. Science Daily. 2009. “Let Me Sleep On It: Creative Problem Solving Enhanced By REM Sleep.”