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What Happens in Your Body When You Fall Asleep

What Happens in Your Body When You Fall Asleep

Have you been struggling with a problem during the entire day only to suddenly find the solution the next day? Did you know why you grow wiser with age? Have you had a long and detailed dream only to wake up and find out only 15 minutes had elapsed?

The answer to all these questions lies in what happens to your body and brain after you fall asleep. If you have wondered why do we need to sleep, you will find the answer over different parts of the article.

Why do we need to sleep

We do not sleep only because of rest

If you have wondered before why do human beings need to sleep, I doubt if you had a clear answer. The best science-based answer you had would stop at “the body and the organs need rest.”

Agreed, but why does the body need rest in the first place?

Why did sleep evolve?

Matthew Walker explains the science of sleep in his book Why We Sleep. He explains how nature might have created sleep as a part of evolution.

Evolution exists to increase the chances of survival of any species. For example, eyes developed so that organisms can view surroundings and save themselves from predators. Hair developed to protect the ancestors of humans in the cold.

Did you know our ancestors had hair comparable to a chimpanzee? Oh wait, we evolved from the monkeys anyway.

Over the years, when humans moved to hotter lands, the hair made the body hotter than necessary. The amount of hair reduced, to cool the body, avoid the extra insulation and to prevent the growth of parasites.

Every part of your body has gone through many iterations of evolution. Science may not know the reasons for each one of them, but the process has happened a million times throughout billions of years.

Amidst all this, why did evolution make us sleep at night? Think about it for a moment. There were no safe houses during ancient times. Sleep made human beings more vulnerable to attack by predators. Therefore, the reason must involve more than just rest. Evolution must have a strong reason to create sleep. If not, sleep would have vanished somewhere over the course of evolution.

Why sleep evolved science

What happens to your body when you sleep?

Sleep includes more than rest. Far more happens in your body when you go to sleep. Your brain does some unbelievable things while you are asleep. Knowing how the brain works at night and how sleep changes your body is mesmerizing. The knowledge about the sleep process will change your perspective on sleep completely.

The 2 stages of sleep:

Sleep comprises of 2 stages, called Non-Rapid Eye Movement(NREM/Deep Sleep) and Rapid Eye Movement(REM) sleep. These do not exactly happen one after the other. Each of these happens in cycles, though some parts of NREM and REM sleep are spread throughout the night.

2 Stages of sleep

Most of the NREM sleep occurs soon after you go to bed while the REM sleep happens during the second half or fag end of your sleep.

How do you grow wiser with age?

To understand how you will need to understand what happens to your body throughout the day. Once you understand how the brain functions through the day and night, the benefit or role of sleep will become crystal clear.

What happens when you sleep



Throughout the day your body captures information using your senses. At every instant, your body receives some form of information or the other. It can be:

  • The sound of your alarm clock
  • The cool breeze your skin feels
  • The color of the wall in front of you
  • The smell of fresh coffee
  • The taste of the burger from McDonald’s

Whether you focus on such information or not, your body is collecting them via the 5 senses, eyes, ears, nose, tongue and skin. All the information captured ends up in part of your brain called the Hippocampus.

This part serves as the temporary storage for all the information captured. Like the RAM of a computer, the Hippocampus holds current details in memory.


When you go to bed, you enter the early part of your sleep called Deep Sleep or NREM(Non-Rapid Eye Movement) sleep. The magic starts when you fall asleep and lose consciousness.


Like you pick your favorite dishes from an extensive buffet, your brain picks only the useful information during NREM sleep.

During the NREM phase, your brain only retains useful details from the reception phase of the day. Your senses have captured a cartload of information throughout the day and most of it does not add any value.

If you have to pick gold dust from mud, you have to discard a lot of mud and filter out the gold. Similarly, your brain springs into action as you enter the NREM sleep to filter out all the unnecessary details.

For example, you might have noticed the color of the car parked outside your house. You might have also learned a new algorithm at work today. Your brain knows that the color of the car is useless while the algorithm is useful. Therefore, the color of the car gets discarded while the brain retains the understanding of the algorithm.

During the NREM phase, the important things in memory get moved to permanent storage. What was previously stored in the hippocampus now moves to the neocortex. Sleep scientists have performed studies on people by analyzing their brain activity before and after sleep.

Once you sleep and wake up, the brain pulls the same memory from different parts of your brain. Before you fall asleep, the algorithm you learned originates from the hippocampus. Once you asleep and wake up, the same algorithm comes from a different physical part of the brain, the neocortex.

The functioning of the hippocampus and neocortex resembles the RAM and hard disk on a computer. The RAM serves the applications running at present. The RAM has limited capacity. Any data which requires long term storage must be saved on the hard disk.


Through the night, you move to the second half of your sleep called Rapid Eye Movement(REM). During this phase, your eyelids move rapidly as a spirit has possessed you. During this phase, time in your brain dilates and moves slowly.


During the REM phase, the details your senses gathered today are replayed in a quarter of the regular speed. During this phase, the brain tries to integrate the new information gathered during the day with the existing knowledge in the brain. The brain tries different combinations by replaying the information gathered during the day at a slower speed.

By connecting all the old and new information, you become a smarter person after you wake up.

During the REM sleep, the muscles in your body are paralyzed. If I lift your hand and let it go, it would drop down like you were unconscious. Your brain intentionally paralyzes your body so that you do not act out the dreams in your head.

For example, if your brain attempts to strengthen your skill in golf without paralyzing your body, you would swing your hands and knock a few teeth off your spouse sleeping next to you.

However, the brain is smart enough to paralyze only your muscles. It does not paralyze all your senses. But it does not keep your senses completely open to information either.

A part of your brain called the Thalamus filters out a lot of information reaching your senses such as hearing, feeling, smell to allow you to sleep without any disturbance. If you hear a soft noise or have someone rub your arm gently or sniff a deodorant sprayed in your room, you will not notice any of them.

However, if there is a loud noise or someone whacks you on your butt or sprays a strong perfume right next to your nose, you will wake up. The Thalamus knows how much information to block. It ensures your body does not wake up due to normal incidents but springs you awake when a massive event happens around you by sensing danger.

If that sophistication and brilliance does not convince you why is sleep important, I am not sure anything else will.

Interesting questions answered

Have you had a long dream only to wake up and notice only a few minutes had passed?

Dream Time dilation

As mentioned earlier, time dilates in your brain during sleep. Since the brain replays information at quarter speed during the REM phase, time seems slower. At the same time, the brain is trying to interconnect old and new information by trying different possibilities. What seemed like an hour in the dream ends up only as 15 minutes in real.

If you have watched the movie Inception where time moves slower in a dream, you will know what I mean.

How do you wake up with a solution to a problem you were struggling with the previous day?

Sleep over a problem

During the REM your brain connects the details from the day with the rest of your knowledge. Your conscious brain cannot easily access all of your knowledge. However, your subconscious brain tries various possibilities during REM sleep and attempts to solve the problem for you.

If you tell yourself that you need a solution to a problem before going to bed, the brain tries finding a solution while you are asleep.

For the same reasons, musicians sleep after practice. During one of Matthew Walker’s presentation, one person from the crowd who was a pianist told him “I often struggle with some notes I try to learn. No matter how much I practice during the day, I cannot get it right. But the next day, I magically seem to play the note without any flaws. It cannot be a coincidence.”

This triggered a thought in Matthew Walker’s brain which led to multiple experiments to understand how sleep helps us learn a skill.

The saying “Sleep over a problem” holds far more truth than you believe.

Do other living things have REM sleep too?

Not all organisms have REM sleep. Some birds and mammals do but fish and insects do. One possible reason could be because REM sleep paralyzes muscles. If the muscles of the fish get paralyzed, it would end up drowning. Therefore, in fish and insects, the body remains slightly active even during sleep.

Why do you find it difficult to sleep in the hotel on the first day?

Cant sleep

As a usual practice, your body goes through Deep Sleep and REM sleep. The brain smoothly goes through both these under familiar conditions only. If you sleep in your house on your bed, your brain knows you are safe.

However, in a hotel, your brain faces unfamiliar conditions. To protect you from danger, your brain allows certain senses to stay awake to sense danger. Due to this extra attention, your body cannot enjoy a comfortable sleep like usual.

If you are a frequent traveler and always stay at the same hotel, your brain trusts the surroundings better. Staying at the same hotels allows you to sleep well from day 1.

How many hours of sleep is necessary?

Sleep necessity

Now that you know what happens while you sleep, you already realize what your body misses if you do not sleep enough.

What is the maximum sleeping time?

9 hours. If you sleep more than 9 hours on average you are sleeping too much. That said, it is perfectly ok to sleep for 12 hours occasionally just because you were feeling lazy that weekend.

What is the minimum time to sleep each day?

6 hours is borderline and is the minimum sleep required. It is like a student passing a subject with minimal possible marks. 7 hours is recommended by most sleep experts.

Maybe you can manage to go through your day sleeping lesser. But sleeping any lesser than 6 hours on a prolonged basis can damage you. Here is what happens if you don’t sleep enough:

  • you could become less intelligent
  • you weaken your immune system
  • you turn more prone to diseases like Alzheimer’s and cancer
  • you end up forgetful, irritated and less productive

Based on sleep cycles and what sleep experts mention, you need a minimum of 7 hours of sleep and a multiple of 90 minutes. By that calculation, 7.5 hours of sleep every day is optimal for adults.

By sleeping less, you inhibit the set of improvements within you which mother nature designed as a part of evolution. Sleeping for fewer hours on an occasional day is ok, but sleeping for less than 6 hours daily impacts your memory by disturbing your NREM sleep, it impacts your wisdom by hampering your REM sleep.

Margaret Thatcher and Ronald Reagan who boasted sleeping for less than 5 hours a day suffered from Alzheimer’s during their later years. While no one has evidence that lack of sleep caused the ailment, scientists have found close relationships between lack of sleep and Dementia/Alzheimer’s.

No concrete proof between the two has been established yet though. But better safe than sorry.

You can calculate the appropriate time to wake up by using this link. It calculates the time to wake up fresh based on sleep cycles.


The subject of sleep and what the body and brain do to you while you sleep is mesmerizing. When you understand the science behind sleep you become aware of a ton of reasons why sleep is important.

Sleep, over the years, has evolved into a masterstroke of nature. Since most people do not know what happens to the body when asleep, they ignore the functions of sleep itself. Sleeping less does more damage to your body, brain, and behavior than you think.

Dont stay awake late

Do yourself a favor and prioritize sleep over browsing social media or Youtube. The average hours of sleep are decreasing with each generation. We are yet to see the perils of sleep deprivation in the current generation.

20-30 years later, the effects will show up. Do you want to be a part of those statistics who suffered problems in later life due to lesser sleep by browsing the smartphone uselessly on the bed? I wouldn’t.

sleep benefits

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