Why Do We Sleep and How Sleep Restores Our Body and Mind

Let’s find out why we need sleep and how it restores our body and mind.

Why Do We Sleep

Humans spend on average one-third of their life sleeping. During that time, our brain and other organs perform certain processes they can’t do when we are awake. These may be like toring memories and repairing cells. All these activities are necessary for our survival and well-being.

As recommended by the National Sleep Foundation guidelines, adults should have 7–9 hours of sleep every night. But this is something often neglected and readily sacrificed. It can have serious consequences for our health. So let’s find out why we need sleep and how it restores our body and mind.  

Reasons why we sleep

Defining sleep is an ongoing process. There are a few theories that try to explain it. But, none of them are accepted as the definition of sleep: 

  • Inactivity Theory believes that evolution taught humans to be inactive at night to avoid injuries from lack of light. 
  • Energy Conservation Theory is based on the notion that humans need to reduce their activities in order to preserve energy necessary for hunting for food. 
  • Restorative Theory explains that our bodies need to sleep for cellular repair and regeneration to take place. 
  • Brain Plasticity Theory states that sleep allows neural reorganization and development of brain’s function. 

Having more than one theory doesn’t mean that none of them are true. But rather that sleep may be explained as a combination of opinions. But undoubtedly, we all need sleep to be productive, focused, mentally healthy, and energized to go about our days. 

Stages of sleep

The sleep cycle has four stages. The first three are called NREM or non-rapid eye movement. The fourth one is known as REM or rapid eye movement. During one sleep period, you can have several cycles, each lasting 90–120 minutes and involving all four stages.

  1. Stage 1 — This NREM stage involves transition into sleep also known as light sleep. At this point, your heartbeat, breathing, and brain waves are slowing down, while muscles start to relax. It usually doesn’t last longer than a few minutes.
  2. Stage 2 — Another NREM stage characterized by deeper sleep when the heartbeat and breathing are still slowing down. There is no eye movement and the body temperature starts to go down. Brain waves are steadily slow, while the muscles become even more relaxed. This is the longest of all four stages.    
  3. Stage 3 — Last NREM stage that defines how refreshed you will feel the next day. This is when the body systems that have been slowing down in the previous two stages are at their lowest. The duration of this stage is not the same throughout the night as cycles alternate and is the shortest as your sleep is coming to an end.
  4. Stage 4 — This is the REM stage characterized by eye movement visible under the eyelids. At this point, the heartbeat and breathing are speeding up, and your blood pressure will start to go up. You will start dreaming and all your limbs will be paralyzed so you don’t react to the events in your dream and hurt yourself. During Stage 4, the brain converts experiences into long-term memories, in the process known as memory consolidation. Each REM will be longer as your sleep progresses, but the duration of this stage will become shorter as you age.    

What happens to the body and mind during sleep

A 2017 study demonstrated that having eight hours of sleep can save 35% of the energy you have when awake. Muscle repair, tissue growth, hormone release, and protein synthesis all happen when we sleep, as well as cellular restoration. 


While brain waves may be slower, nerve cells reorganize, and memories are sorted out from short-term to long-term. This is also when the brain removes waste and toxic byproducts that accumulated during the day through its lymphatic system. Sleep is important for proper brain functions, like problem-solving skills, learning, creativity, focus, and the decision-making process.

Strong immune system

The immune system is our line of defense against pathogens that cause diseases. Antibodies, cytokines, and other immune cells are produced during sleep, so not getting enough shuteye can leave you vulnerable to infections. It’s a well-known piece of advice given by doctors: sleep well and rest when you are sick so your body can fight off germs.   

Weight control

Hunger hormones are also affected by the quality of sleep. Since you are using less energy during sleep, ghrelin levels, the hormone responsible for appetite increase, will be lower. In case you don’t sleep enough, levels of leptin, the fullness hormone, will decrease and ghrelin will increase. Because of this, you will eat more and gain weight, raising the risks of type 2 diabetes, obesity, and metabolic syndrome.

Emotional health

Another aspect that depends on a good night’s sleep is emotional well-being. When you get enough sleep, you are more likely to react to situations in an adaptive way, but if you are sleep-deprived, chances are you will overreact. For example, you can get easily agitated, prone to anxiety, and overemotional due to lack of sleep.     

Things that can ruin a good night’s sleep

Clutter in the bedroom, outside noise, an uncomfortable bed, and ambient light can be the reason why you are not able to fall asleep. To make the bed more comfortable, use cotton sheets and mattress protector with appropriate characteristics, like with an anti-allergic or a cooling effect. A tidy room has a calming effect on the mind, unlike unease caused by clothes and other things being all over the place. 

Every human has an internal clock known as the circadian rhythm that tells the body when it’s time to go to bed and wake up. If you are exposed to light when in bed, it can make your body believe it’s daytime and prevent you from falling asleep. This is why you should keep electronic devices, like smartphones and tablets, outside the bedroom since the blue light from the screen can disrupt your circadian rhythm.

How to improve sleep

Besides creating a comfortable environment that will appeal to your circadian rhythm, there are other things you can do to improve sleep. First of all, you should quit bad habits, like smoking. Nicotine withdrawal happens during the night when you go to bed and can’t smoke, which can make it difficult to fall asleep and even cause insomnia. 

Caffeine is another stimulus, like alcohol and nicotine, that can be responsible for a poor night’s sleep. If you can’t quit drinking coffee, limit yourself to having it early in the afternoon the latest and try decaffeinated options. 

Being physically active is known to help you sleep better, especially if you exercise regularly. However, make sure not to work out close to your bedtime since instead of helping you fall asleep, it can keep you awake. Meditation and breathing techniques may help you eliminate stress and feel calmer before going to bed. 

Catching up on lost sleep

It’s acceptable to sacrifice a few hours of sleep occasionally but don’t make it a habit. Occasionally lost sleep is easy to recover by sleeping in the next day. However, a 2016 study showed that to completely recover an hour of lost sleep, you will need four days. That being said, you can pretty quickly experience a sleep deficit if you make it a habit to spend less time sleeping. 

The bottom line

Understanding why we sleep and how it restores our body and mind is the best approach to take this physiological process more seriously. Taking care of our health should be a priority and that includes getting the necessary hours of sleep. Doing everything possible to sleep better may lead to some hard decisions and changes of habits, but it’s worth the trouble.