Why Do We Need to Sleep?

Are you — like every other person — mystified by the subject of sleep? Do questions like “Why do we need to sleep?” pop into your mind very often? 

You are not alone! Even the best scientific minds still haven’t figured out why humans need to sleep. All they know is that night of sound sleep rejuvenates and invigorates our mind and body. It helps us recover from tiredness after a long day and makes us feel brand new the next morning. Not to forget, other essential processes like memory consolidation, muscles/tissue repair, or the release of the growth hormone occur while we sleep.

But why do we need to sleep? Why can’t our bodies carry out the functions mentioned above during the wakeful state? 

I’d like to call your attention to the fact that humans spend about a third of their lives asleep. The average lifespan of a healthy person is 79 years. So, almost 27 years are spent sleeping. Imagine the things we could do if we didn’t need to spend 27 years of our lives asleep? It’s nearly three decades wasted doing nothing!

What Do We Know About Sleep?

We know that human beings can only afford to spend a limited amount of energy in a 24-hour cycle. Crossing over the limit would result in tiredness, fatigue, exhaustion, and drowsiness. 

So, it would be fair to assume that our body can’t stay awake and recharge at the same time. This is also the reason why in a 24-hour cycle (also known as circadian rhythm), an individual stays awake for 16-18 hours and sleeps for about 6-8 hours.  

The sleep/wake cycle is observed in animals too. In a research paper published by PLOS Biology, two sleep experts, Cirelli and Tononi, termed the failure of scientists to identify the core function of sleep as a “null hypothesis.” While working on the null hypothesis, Cirelli and Tononi were able to list (regarding their sleeping patterns) living beings into three categories: 

1) animals who do not sleep at all 

2) animals that can go without sleep for an extended period

3) and finally, those who can circumvent the negative consequences of sleep loss.

What I’m leading up to is this: No animal can go without sleep. There are indeed creatures with sleeping patterns different than that of human beings. Some sleep more than humans, whereas there are a few who rest for 1-2 hours. But none can go without sleep

Some people claim to have found animals that do not sleep at all. Take bullfrog; for example, for years, it has been promoted as an animal that does not sleep. But the research performed to validate this claim is inclusive. 

Digging further in the study by Cirelli and Tononi, we found many interesting things about sleep. For instance, Dolphins are the only creature on earth that can have half of their brain sleep at a time, and the other half wide awake. This phenomenon is known as unihemispheric (one-sided) sleep. 

However, several research organizations have raised issues concerning one-sided sleep in Dolphins. In a nutshell, most researchers do not consider one-sided sleep as ” real sleep.” Just because a part of the Dolphin brain does not function, does not mean it is partially asleep. We hope you now know quite a bit about how sleep functions in various organisms.

What Happens During Sleep?

No wonder why sleep is so mystifying for someone knowing nothing about the sleep cycle or the internal biological clock. Your mind begins to get tired at a particular time of the day (somewhere around 9-10 PM), and you go to bed following your internal urges. 

The entire duration, while you are asleep, feels like a blip. Next thing you know, it’s morning, and you wake up renewed and ready to go.

But things are not that plain and simple. Although the primary reason behind why we need to sleep is not yet found, scientists have identified the four main stages that a person goes through during the sleep process. Read further to find out about these four stages. Also, refer to the flow chart in the following image. 

Why Do We Need to Sleep
Image source- wikipedia

Stage 1: 

This is the stage when you make the transition from a fully conscious state to being unconscious. Now, the time each individual takes to slip into “stage 1” depends on a lot of factors. 

You don’t fall asleep the moment you lie on the bed. You fall asleep depending on how sleepy you are, and external factors like surrounding noise, the temperature of the room, etc.

Once you enter the “stage 1,” your breathing and heart rate begin to slow down. Also, your muscles get more relaxed and loose. But you can be easily awakened by external noise or physical touch. After about 20 minutes of semi-unconsciousness, you enter into stage 2. 

Stage 2:

Stage 2 (also called a light sleep stage) is quite similar to the first stage. The only variance is in the degree of unconsciousness. 

You sleep deeper in this stage. Also, brain wave activity slows down a bit, but it is still active, and you can be awake and able to see, hear, and think within no time. After about 20 minutes of being in “stage 2”, you enter into the next stage.

Stage 3:

In this stage, you enter into a deep sleep. Your breathing and heart rate slows down even further. And your muscles become entirely sedentary. Plus, your body temperature drops by a few degrees. 

In this stage, factors like external noise or a physical touch won’t wake you up quickly. You are less likely to get awakened by your phone ringing or someone bumping into you inadvertently. Lastly, this stage lasts a bit longer than the first two (about 30-35 minutes). After that, you enter “stage 4.”

Stage 4:

In stage 4, you sleep even deeper, and your body temperature falls even below what it was in the previous stages. Moreover, your blood pressure also lowers. 

In this stage, the Growth hormone (GH) is released into your bloodstream by the pituitary gland (located in the brain). This hormone promotes muscle growth and also speeds up the metabolism of protein, carbohydrates, and fats in your body. This stage lasts for about 20 minutes.

After going through the four stages mentioned above, your sleep cycle goes in the reverse direction. It goes back to stage 3, stage 2, and finally enters (skipping stage 1) REM (Rapid Eye Movement) stage. 

REM is the stage where people experience vivid dreams. It should be noted that all four initial stages are NREM (Non-Rapid Eye Movement). 

Why Do We Need to Sleep? 

As we mentioned earlier, scientists are still searching for a definitive answer as to why we need to sleep. It is understood that humans and other living beings like animals, bacteria, fungi, etc. need to sleep to recover and revive from the mental and physical energy losses that occur during the day. 

Although sleeping patterns in humans are a lot different than that of other animals, the primary purpose of sleep is the same. So, as of now, we can safely assume that humans sleep at night so they can wake up feeling rested and refreshed.  

According to the 2nd edition of Neuroscience, a book by Purves D, Augustine GJ, and Fitzpatrick D, sleep requirements in infants and teenagers are much higher than average healthy adults. Infants sleep about 16 hours a day, whereas teens need 9-10 hours of sleep each night.

Why Do We Need to Sleep
Image Source- ncbi

The above two graphs should make the picture clear for you. The first diagram (A) shows the duration of sleep, and the (B) depicts the period of daily sleep as a function of age. 

In the above two diagrams, you can see how sleeping patterns in a human being changes from infancy, throughout adulthood and up to old age. Humans need more sleep in infancy and old age. 

How Long Can We Survive Without Sleep?

Now that you know how important sleep is in a living being, we won’t be surprised if questions like “how long we can survive without sleep?” pops up in your head. It is not easy to come up with a straight answer to this question. It is quite dicey to experiment with sleep loss as it could lead to sudden death. 

We found several reports where a person tried to go sleepless for as long as they could and ended up dead after just a few days. Remember how the “King of Pop” Michael Jackson died?

In case you didn’t know, Jackson did not die from a drug overdose. There is a myth that he OD’d on painkillers while preparing a major show. Yes, Jackson was under the influence of a sleep-inducing drug called “propofol,” but he did not die from it. He died as a result of not sleeping for 60 days straight. 

A science report states that propofol suppresses rapid eye movement (REM) sleep. While Jackson did benefit from the drug — it made him fall and stay asleep at night — it also prevented him from entering the REM stage of the sleep cycle. Deprivation of REM sleep turned out to be fatal.

To answer your question, there is no specific limit up to which a person can go without sleep. Although a man named Randy Gardner holds the record for the longest a human has gone without sleep. This 17-year-old, in the year 1965, managed to stay awake for 11 days and 25 minutes as he worked in preparation for a high school science project fair.


Sleep isn’t just about resting for 6-8 hours unconscious on the bed; it is much more than that. The science of sleep helps you understand the different stages involved in it. 

Knowing these intricate details enables you to optimize your sleep. As a result, you are more productive and creative the next day. 

In this post, we have covered most of the topics linked to the question of “why we need to sleep.” Let us know if you have any further queries related to sleep.