What is Sleep?

By Mark Thompson

Dec 13th, 2022

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Although at first glance it might seem obvious what sleep is, it remains tantalizingly difficult to pin down in definitive terms. We might be tempted to specify that sleep is a passive state of unconsciousness, a suspension of our normal bodily activities, as was assumed for millennia. 

However, we now know that this is far from the truth, and that sleep is actually a complex and far-from-passive process of active internal restoration, recuperation and reconsolidating which is essential for our health and well-being.

Moreover, it is hardly possible to talk of such a simplistic thing as “sleep“. Our nightly sleep is made up of several sleep cycles, each of which is composed of several different sleep stages, and the physiological and neurological differences between the two main types of sleep, rapid eye movement (REM) and non-REM, are almost as profound as the differences between sleep and wakefulness.

We might not have all the answers about what sleep is, why we do it, or what happens while we’re asleep, but there is still plenty of sleep research worth exploring. Find out how much sleep you need, what your dreams mean, and what happens when sleep goes wrong in the sections below.

What is Sleep?

Sleep is one of those things that is so natural to humans, that it’s actually hard to pin down a definition. But we’re going to give it a shot. First, we know that when humans go to sleep, their conscious mind shuts down.

Usually, our eyes are closed and our bodies are relatively still, though again, there are exceptions. While asleep, our brains cycle through several different stages of sleep, from light REM sleep to deep sleep. After sleeping, our bodies and minds are supposed to feel rested and restored, although certain health conditions like chronic fatigue syndrome or depression may prevent that from happening. And we know that all humans need sleep, even though we aren’t entirely sure why. 

Based on these facts, we can come up with a working definition of sleep. Sleep is a necessary human activity characterized by unconsciousness and temporary suspension of voluntary muscle movement, made up of several sleep stages, that results in mental and physical restoration. It might seem a little wordy, but that’s because even though it feels simple, sleep is a complicated subject.

Stages of Sleep

As we mentioned before, sleep is broken up into multiple stages, which differ from each other based on eye movement, heart rate, brain waves, body temperature, and other physical signs.

sleep stages

The first stage of sleep is called Stage N1 sleep. This is a form of non-REM sleep and it is a very light sleep stage where you may still feel half-awake. After N1, our brains go into Stage N2 sleep, which is still a light, non-REM sleep, but you feel fully asleep at this point and it becomes harder to wake you up from Stage N2 sleep.

Next, your brain enters Stage N3 sleep, also called slow-wave sleep (SLS). This is the last non-REM sleep stage, and it is the deepest stage of sleep. In this stage, you are very hard to wake up, and your brain is doing lots of hard work storing energy for the next day and processing memories from the day before.

Finally, your brain will jump to REM sleep, which most people know as the “dreaming stage.” But the truth is, we can actually dream in non-REM sleep as well. REM sleep is a very light sleep stage with significantly increased brain activity compared to other, non-REM stages. It’s actually very close to waking brain activity, but in REM sleep, our bodies utilize a kind of temporary paralysis to keep us from walking around in our sleep, acting out our dreams (issues with this paralysis system can cause parasomnias like sleepwalking).

The Circadian Rhythm

Most jobs require us to get up early, and if we want to get a decent amount of sleep, that means we need to go to bed early too. But if you didn’t have to get up for work, what time would you naturally wake up and go to sleep?

That natural timing, your own personal “internal clock” is called your circadian rhythm, and scientists believe people who consider themselves “night owls” may have circadian rhythms that differ from the norm.

Circadian rhythms are actually quite complicated and depend on a number of different factors. First, your body’s natural production of melatonin plays a huge role when you start to get sleepy. For most of us, melatonin production increases in the evening and reaches its peak in the very early morning, but for people with delayed sleep phase syndrome (DSPS), this melatonin production may be behind schedule by two hours or more.

Sunlight also plays an important role in our natural circadian rhythms, which is why moving to an area near the North or South poles could mess with your sleep patterns. These areas tend to have months of extra sunlight or practically no sunlight, which can throw a real wrench in your internal clock.

How Much Sleep Do We Need?

Odds are, you know that you’re supposed to get 8 hours of sleep each night, but did you know that this isn’t true for everyone? For instance, we know that babies need far more than 8 hours of sleep, and older adults actually need a little less sleep. These differences are largely due to differences in circadian rhythms.

Babies don’t have set circadian rhythms at birth, and instead they tend to sleep in several chunks throughout the day (known as polyphasic sleep), while seniors experience a phase shift in their circadian rhythms causing them to get sleepy earlier and wake earlier than they did when they were younger.

If you notice that you tend to feel your most alert and awake after getting only 6 hours of sleep, or if you find that you experience daytime sleepiness any time you get less than 10 hours of sleep, you may be one of the exceptions to the 8-hours rule. Approximately 6-10% of people need more than 8 hours of sleep each night, while around 5% of people actually function better with a little less than 8 hours of sleep.

Everyone is different, but we all need a good amount of sleep.

What Happens When We Don’t Sleep? 

We may not totally understand what happens when we sleep, but we definitely know that we need it. Without sleep, humans experience some serious repercussions, both in the short- and long-term.

Short-term, sleep deprivation can cause sleepiness, distractibility and lack of focus, and these things can be deadly. Studies show that driving tired is just as bad as driving with a blood alcohol content of 0.10, which is about one drink over the legal limit.

And even though the term “sleep debt” makes it sound like you can make up for your sleep deprivation by simply getting more sleep later, it doesn’t really work that way. There are long-term issues with missing out on quality sleep as well.

Long-term, sleep deprivation can have serious consequences for your health. Lack of sleep can lead to heart disease, decreased immune system function and weight gain. So whenever you can, make sure you get enough sleep.

Sleep Disorders

So far we’ve discussed what sleep is supposed to look like, but what about when sleep goes wrong? Sometimes we may experience a brief sleep disturbance, but other times, our sleep problems become a real issue and may qualify as an actual sleep disorder, also known as a parasomnia.

Some common parasomnias include:

  • Delayed Sleep Phase Syndrome: When a person’s natural circadian rhythm doesn’t encourage sleepiness until much later than normal.
  • Insomnia: When a person experiences a consistent inability to fall asleep.
  • Narcolepsy: When a person experiences uncontrollable daytime sleepiness or sleep attacks.
  • Obstructive Sleep Apnea: When a person’s breathing is obstructed by an physical blockage in the airway.
  • Restless Legs Syndrome: When a person experiences uncomfortable sensations in the legs, especially before bed or throughout the night.
  • Sleep Paralysis: When a person wakes up while their body is still paralyzed from REM sleep, and may experience disorienting hallucinations.
  • Sleepwalking: When a person gets up and walks around in their sleep without waking up.

Why Do We Sleep?

As we’ve mentioned already, scientists really aren’t 100% sure why we sleep, but there are a few very fascinating theories out there. All of them actually have some decent evidence to support them, so in reality, the reason we sleep might be a combination of all of the theories.

The three prominent ones we want to highlight are sleep for survival, sleep for growth, and sleep for memory processing.

Sleep for Survival

According to this theory, people and animals need sleep because it protects them from danger in some way. This theory suggests that we sleep during the hours that are most dangerous in order to avoid attracting predators with movement and noise.

However, this theory doesn’t explain everything, like why we’re so defenseless when we sleep, or why some high-ranking predators with few threats to worry about, like lions and humans, sleep such long hours.

Sleep for Growth

This theory says that we sleep because it helps us grow, especially slow-wave sleep. Studies show that children and athletes spend more time in slow-wave sleep than older adults, suggesting that more slow-wave sleep is necessary for cell regeneration and repair (AKA, growth).

Sleep for Memory Processing

This last sleep theory suggests that sleep exists to help us process our memories, and there is a lot of science to back it up. A good night’s sleep has been linked to better procedural memory (memory of how to do things) and better declarative memory (memory of facts and events). Plus, sleep seems to play an important role in helping us transfer information from short-term to long-term memory.


If you thought scientists know shockingly little about sleep, then prepare yourself. We know even less about dreams. We used to believe that we only dreamed during REM sleep, but it turns out we dream during non-REM sleep as well. And as for what purpose dreams serve- well, we’re guessing there too.

According to our best theories, dreams could be a completely random collection of images caused by random firings in our brains as we sleep, or like sleep itself, they could help us process and store our memories. Other scientists suggest dreams are meant to help us survive, by simulating threatening situations so we can unconsciously prepare for them in case they happen in real life. The truth might be a combination of these theories, or maybe none of them at all.

Dreams are also strange because they mess with the very definition of sleep. Remember our basic definition of sleep? We said that sleep meant our conscious mind was basically temporarily out of order, but lucid dreaming challenges that definition. Lucid dreaming is when a person is aware that they’re dreaming, and sometimes they can even control what happens in the dream.


So we don’t know everything about sleep just yet, but we do know that getting enough sleep is important for our physical and mental health, when we sleep is largely determined by melatonin and sunshine, and dreams are very weird. If you are concerned about your sleep problems and think you may have a sleep disorder, you can find more resources here on Mattress Advisor.