What Happens When You Sleep?

If you have ever been interested in what happens to your mind and body when you sleep, you aren’t the only one. Scientists have spent a considerable amount of time and effort in trying to discover precisely what is going on for the eight hours or so that we are virtually unconscious.

While there is still a lot that we don’t know, modern science has managed to find out quite a bit about what takes place during your slumber:

NREM and REM Sleep

Sleep can essentially be divided into two main categories. These are known as Rapid Eye Movement (REM) and Non-Eye Rapid Movement (NREM) sleep, with NREM consisting of three different stages. These are known as N1, N2, and N3. Overall, each person goes through NREM and REM in a specific order until they wake up.

When you first fall asleep, you are entering into the NREM portion of your cycle. In fact, you will spend about 75 percent to 80 percent of your slumber in this phase. After going through the first sleep cycle, you will begin to go into the REM phase.

This stage is identified by the rapid side-to-side movement of your eyes underneath closed lids.

Unlike adults, babies spend up to 50 percent of their slumber in REM sleep.

Stages of Sleep

So, as it was mentioned in the part above, sleep can actually be separated into distinct sleep stages. Your brain and body react in different ways, depending on the phase that you are in. Here we will go into greater detail of the sleep stages 1 – 4.

Stage 1

Stage 1 or N1 is where you are making the shift from conscious to sleeping. So, this part is marked by a very light sleep that lasts no more than ten minutes which makes up just 5 percent of your total sleep. If you were to be woken up at this point, there is a good chance that you may not even realize that you were sleeping.

This is also the time that you are most likely to experience the feeling of falling and wake up. You may also be startled awake by something known as a hypnagogic jerk, which is a muscle spasm.

As you begin falling asleep, there are various changes happening to your body. Your breathing begins to slow down, as does your heart rate. It is at this time that your overall body temperature also begins to lower and your muscles start relaxing. Even your brain waves begin to slow.

Stage 2

At the beginning of the night, you only spend a little longer in sleep stage two than you do N1. However, as your cycle progresses, you will end up spending the majority of your slumber in this stage. At the same time, this is a phase when you sleep fairly lightly.

All of the physiological changes that began to take place in N1 continue in N2 as well. So, your muscles will relax even more, as will your heart rate and brainwaves. Any eye movement that was happening before, will come to a stop.

Stage 3 and 4

Initially, when scientists were still figuring out how sleep worked, they separated stage 3 and stage 4 sleep. However, since there wasn’t too much that could distinguish these phases, they began to refer to it as N3. This is the deepest stage of sleep which is why it can be very difficult to wake someone up from this stage.

It is during this stage that your heart rate, blood pressure, and breathing drops to its lowest levels.

Despite this, they are all still within healthy ranges.

Your body temperature also drops even further and your muscles are a lot more relaxed as well. You can spend up to 40 minutes at a time in this stage.

REM Stage

time and sleep

You go into REM sleep around ninety minutes after you have fallen asleep. It is theorized that it is in this stage that people dream the most. Experts also assume that it is this dreaming that leads to the greater amount of eye movement seen in this phase.

Something quite interesting happens to your body during the REM stage. While your temperature reaches its lowest levels, other vital signs begin picking up. Your heart rate increases and your breathing becomes shallower.

Your brain also becomes incredibly active – in fact, many scientists believe that your brain is more energetic during this period than when you are awake! This, of course, probably has to do with the fact that you are dreaming.

The one part of your body that isn’t all that responsive during REM sleep are your muscles and your limbs. They become incredibly relaxed and are difficult to move. This is largely so that you will be unable to act out your dreams while you sleep.

Your Brain While Sleeping

When you go to sleep, most of your body gets some rest too. As the above section shows your breathing, heart rate, and various other vital signs slow down. Well, one part of your body that doesn’t get any respite is your brain. Even when you are fast asleep, it is still working.

The hypothalamus plays a large role in sleep. This is because it contains a cluster of nerves known as the suprachiasmatic nerve (SCN). The SCN is the reason that your brain signals your body that it is time to go to sleep at night – it is regulated by light exposure. The pineal gland is signaled by the SCN to begin producing melatonin which is what aids your sleep.

Another part of the brain that involved in sleep is the brain stem, which works with the hypothalamus to produce chemicals that reduce certain brain activity. The brain stem is also what causes your muscles to relax when dreaming.

There are other parts of the brain such as the basal forebrain, thalamus, cerebral cortex, and even the amygdala that are involved in sleep. As you can see, the brain’s work is never really done.

Why Do You Dream?

The short answer to why do you dream would be: no one, really knows. There are plenty of theories about why we undergo this process but there is no scientific evidence that actually backs it up. There are some hypotheses that imagine that dreams simply occur because certain parts of your brain are getting rid of unwanted information.

There are others still, who follow Antti Revonsuo’s way of thinking about the threat simulation theory. This states that dreams are a way for you to put your survival instincts and skills to the test while you are sleeping. It could be so that you are better prepared to face real-life threats. This, to some extent, has shown results. There is certainly no denying that dreams and creativity have a strong link. Many people have found solutions to their problems or woken up with new ideas for projects based on their dreams.

All of this brings about the question, what are dreams? Dreams can be described as either stories or visual images that you experience while you are asleep. In many ways, a dream can be comparable to something that you experience when you are awake. This is because, for all intents and purposes, a dream can feel real. After all, you probably have been emotionally invested in a dream and nightmare and have been subjected to real emotions. From what scientists can gather, most of your dreaming takes place during your REM stage. Incidentally, this is when the brain is most operational as well.

The Role of Sleep in Memory and Learning

Memory and learning can basically be broken down into three processes – acquisition, consolidation, and recall. Acquisition refers to the experience that you are having and how your brain processes this information. Consolidation is when this experience gets moved into long-term memory storage. Recall, of course, is when you call upon the stored memory at a later time. Now, as you can imagine, acquisition and recall take place while you are awake. Memory consolidation, however, is something that happens when you sleep.

There is a lot that scientists don’t understand about learning and memory. Despite this, there are clear connections in the way that sleep impacts these processes. Essentially, while you are asleep, various regions of your brains are working to create connections and remember events that happened while you were awake.

As a result, sleep could make you more effective at learning things.

What Happens to Your Body While You Are Sleeping

Your body also goes through numerous changes when you sleep. Some of these are observable and may be familiar to you while others are more subtle. Here is a breakdown of what is going on:


moving in sleep

Most people reposition themselves at least a little while they are asleep, with rolling being one of the more common movements. There may be some biological function behind this as it allows the blood to properly keep circulating all around the body.

Now, there are some people who tend to move in their sleep quite a bit. This, however, takes place when you are in deep sleep. When you are in the REM stage of your cycle, something quite interesting takes place.

The body utilizes two chemical systems – the metabotropic GABAB receptors and the GABAA/glycine ionotropic receptors to make sure that your stay put while dreaming. Various chemicals in the body cause your limbs to essentially be paralyzed for a short period of time. This prevents you from acting out your dreams and hurting yourselves or others.

Heart Rate

One of the reasons that your body could need sleep is so that it gives your heart the opportunity to get some rest. From the time that you begin to fall asleep, your heart rate begins to slow down. Now, it can be difficult to pinpoint your heart rate when you sleep since it can be so varied.

Most average people have a resting rate of between 60 to 100 beats a minute.

Also, the deeper that you fall asleep, the slower that your heart rate gets. Still, it is unlikely that your heart rate drops all that much below the average resting rate.

As we already covered, your heart rate does begin to spike up during REM sleep, probably due to the dreams that you are having. During this time, it can be difficult to predict how your heart rate will behave due to constant changes. Despite this, your heart rate will still remain in the range of what is considered normal.



After you have fallen asleep, your oxygen levels begin to dip slightly while your carbon dioxide level increases a bit. This happens because your breathing rate becomes slower during slumber. When you are in NREM, your breathing actually becomes quite regular and it isn’t prone to variation.

Your breathing changes once you enter the REM phase, though. Not only does your breathing get faster, you also take more shallow breaths. This can be for a number of reasons although it’s most likely due to the fact that your rib cage doesn’t move as much during this time. Also, your throat muscles tend to be a bit more restricted as well.

Body Temperature

Your body temperature begins to decrease even before you go to sleep. It is a way of your body beginning to prepare itself to go to sleep. The colder you are (within reason, of course) the likelier you are to sleep well. To aid you in this endeavor, your body actually allows you to lose heat. During the night, you will find that you are about a degree or so cooler than at any point during the day.

Believe it or not, this actually serves a purpose. As your body is allowed to cool, you waste less energy trying to maintain it. This gives several biological systems a rest while you sleep and allows you to conserve energy.

Blood Pressure

Your blood pressure behaves in a similar pattern to your heart rate at night, when you are at rest. So, it dips down during NREM but then spikes back up again during REM sleep. There is also a considerable amount of variation during this stage as well. In fact, it is quite similar to what might be seen during your daytime activity.


If you ever told that you grow while you sleep, then you will be happy to know that it is actually the truth. There are growth hormones secreted during your sleep, which is what allows you to grow as a child. As an adult, however, this hormone has various other physiological roles to play in the body.

Your body takes advantage of you sleeping to make more thyroid hormones to use up during the day. Also, to make it easier for your body to rest, the cortisol levels in your system go down. Cortisol is often referred to as the stress hormone and its decrease leads to a more restful sleep.

Leptin and ghrelin are hormones that regulate your hunger. This can determine when you feel hungry and even the type of foods that you choose to eat. When you get enough sleep, the balance between this two hormones is maintained. Lose several hours, though, and you could end up on an eating binge.


Your body chooses to heal itself while you are sleeping. It is during these seven hours or so that your cells will undergo repair. This includes any injuries that you might have experienced during the day. Basically, though, it is a way for your body to rejuvenate. Despite what most people think, though, there is no proof that your body heals any faster if you get more sleep.

So, getting in some extra shuteye when injured isn’t really going to do much.

Sleep Walking and Talking

sleep walking

While your body might essentially be unconscious during your nap, it doesn’t mean that you are doing things in your sleep. In fact, you may have noticed that there are some people who will talk or walk while asleep. There have even been instances of people during household chores and even driving during this time!

Sleep talking tends to be quite common although, surprisingly, it isn’t linked to the REM phase. You can babble away during any sleep stage. Also, it is usually not a sign of anything serious but stress, medications, and certain medical conditions can increase the chance of you sleep talking.

What isn’t as widespread is sleepwalking. This condition can be described as a combination of your sleeping and waking state. So, although you are technically asleep and aren’t aware of what is going on, your body is able to function as normal. Two of the most popular causes of sleepwalking is a lack of sleep and genetics.

Well, this is a comprehensive explanation of what really goes on when it is lights out for you. This should give you a better understanding of why you need a good night’s rest.


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