You Don't Really Understand Sleep until You Read This

It is widely known and accepted that the time human spend on sleep accounts for about one third of their lifespan. That’s why sleep are catching more attention in modern society. While most people understand sleep matters a lot to their health and life, they do not really understand what sleep is and how it matters. In this page, we will give a brief statement which could help readers know sleep in a more comprehensive way so as to get a better sleep.

1. Sleep Stages

Sleep is not uniform. Instead, over the course of the night, the total sleep is made up of several rounds of sleep cycle, which is composed of four individual stages. In a typical night, a person goes through four to six sleep cycles. Not all sleep cycles are the same length, but on average they last about 90 minutes each.

Sleep cycle refers to a biological rhythm in sleep. People usually go through a whole cycle in about 90 to 100 minutes.

Normally, sleep cycle is divided into two phases: non-rapid eye movement sleep (NREM) and rapid eye movement sleep (REM). NREM sleep and REM sleep appear alternately, and the alternation is called a sleep cycle. There are usually four to six sleep cycles per night, and each cycle is 90 to 110 minutes. The sleep time varies in people at different ages. Infants usually need more than 16 hours on sleep.  Young children need about 12 hours. School children need 9 to 10 hours. Adults need 7 to nine hours. And elderly people only need 6 to 8 hours. But people above 80 years old may need more time (9 to 10 hours) on sleep.

The biological rhythm in sleep refers to a cycle with 5 different stages in about 90-100 minutes. A complete sleep cycle consists of 5 stages: wake, N1, N2, N3, and R. Stages N1 to N3 have considered non-rapid eye movement sleep, each progressively going into a deeper sleep. Sleep is staged in sequential 30-second epochs, and each of these epochs is assigned a specific sleep stage. The majority of sleep is spent in the N2 stage.


The first stage is the wake stage or stage W, which further depends on whether the eyes are open or closed. It is the very beginning of sleep. The feeling of drowsiness belongs to this stage. In this stage, the brain wave begins to change, the frequency gradually slows down and the amplitude gradually decreases.

N1 (Stage 1)

This is the beginning of formal sleep and also the lightest stage. During this stage, brain waves gradually become irregular, and the frequency and amplitude fluctuate. It’s a stage of falling asleep, in which people are easily waken up. But as it just lasts 1 to 5 minutes, consisting of around 5% of the total cycle, people can quickly sleep deeper as long as they survive in the 5 minutes.

N2 (Stage 2)

As heart rate and body temperate drop, the sleeper enters the next sleep stage, which features periods of muscle tone (muscles partially contracting) mixed with periods of muscle relaxation. Brain waves become slower, so do brain activities. And the eyes stop moving. Stage 2 sleep lasts about 25 minutes in the initial cycle and lengthens with each successive cycle, eventually consisting of about 50% of total sleep.

N3 (Stage 3)

This is considered the deepest stage of sleep and is characterized by a much slower frequency with high amplitude signals known as delta waves or slow waves. So stage 3 can also be called delta sleep or slow-wave sleep (SWS). The sleeper in this stage is hard to be waken up. Also this is the stage when the body repairs and regrows its tissues, builds bone and muscle, and strengthens the immune system. Most of deep sleep occurs during the first half of the night when N3 stages commonly last for 20-40 minutes. As sleep continues, these stages get shorter, and more time gets spent in REM sleep instead.

REM Sleep

In this stage, brain activities raise up and the EEG is similar to an awake person. Conversely, the skeletal muscles are experiencing atonic and without movement, with only two exceptions: the eye and diaphragmatic breathing muscles that still remains active. REM stage usually starts 90 minutes after falling asleep, and each of the REM cycles gets longer throughout the night. The first period typically lasts 10 minutes, and the last one can last up to an hour. And it will totally account for 25 % of sleep in adults.

In addition to changes in brain waves, the sleeper’s eyeballs show rapid movement (REM). If awakened at this time, most people report that they are dreaming. Therefore, REM has become an important basis for psychologists to study dreaming.

2. Why We Need Sleep

Sleep helps us in many ways.


For children and young adults, deep sleep (sleep that is harder to wake up from) is more beneficial for their growth. During this sleep, the body releases growth hormone. The body also increases the production of protein, which is needed by our cells to grow and repair damage.

Nervous system function:

Sleep deprivation can affect our memory, performance, and ability to think. If one is severely sleep deprived, he/she may even experience neurological problems such as mood swings and hallucinations. Sleep also helps our nerve cells that can repair themselves so as to function optimally. There is also a chance to create some neural connections, improving our brain function and thinking abilities.


Researches on¬†why sleep is so important¬†is not sufficient. But¬†studies¬†in animals have shown that ‚ÄúREM sleep is crucial for proper metabolism and immune function in the liver and synaptic potentiation in the brain‚ÄĚ. Lack of sleep may damage the immune system, which protects bodies¬†from infection.


People who don't get enough sleep are more likely to get their health damaged and have problems like obesity, diabetes and heart disease.

3. How Much Sleep You Need

Generally speaking, a reasonable sleep should be 7-9 hours per night, which is the guarantee of sleep quality. And 7-9 hour sleep a night is exactly the regular sleep time needed from falling asleep to waking up. After adults fall asleep, the pituitary gland first increases the secretion of growth hormone, which promotes cell metabolism, restores physical strength, and stores physical energy. This process takes about 80 to 90 minutes.

Studies have shown that people who go to bed early and get up early are less stressed and perform better in mental health than those who often stay up late. A scientific sleep should be falling asleep from 22:00 pm to 22:30 pm and entering deep sleep half an hour or an hour later, the best time for the human body to naturally enter deep sleep so as to work with full energy in the next day.