In spite of the fact that sleep is a kind of activity that all of us experience every day throughout the whole life, it is still one of the most mysterious phenomena for people. Nowadays sleep is defined as a natural periodic state of rest for the mind and body, in which the eyes usually close and consciousness is completely or partially lost.
Sleep is necessary for human organism, because when we sleep, our body systems rest and restore their strength. At the first glance, sleep seems to be a period, when the whole organism is inactive. Still, there is one exception here: human brains are proved to be very active during the sleep. It was only in the middle of the 20th century, when scientists managed to discover and measure special waves, which are produced by brains during the sleep.
As the result, a new approach to sleep understanding appeared and it revealed quite amazing facts on sleep in general and on how exactly people sleep. Scientists found out that when we sleep, our brains pass 5 phases, which change each other cyclically. Each of these phases is characterized by the specific brain activity pattern and duration.
Drowsiness is said to be the first stage of sleep. It usually lasts from 5 to 10 minutes and is characterized by decrease in heart rate and breathing. A person may be easily awakened from this stage.
Stage 2 is the so-called light sleep. Brain waves become slower, with periodical peaks of rapid waves called sleep spindles. These peaks define the moments of muscle tone and relaxation. Stage 2 is the preparation stage for the human organism to enter the next stages 3 and 4, which are called "deep sleep".
The stages 3 and 4 are characterized by special waves, called delta waves, which are slow rhythmic waves of high amplitude. It may become a real challenge to awake a person, when he/she enters the stage 3 and especially stage 4 of the sleep, because people usually do not respond to auditory or physical irritants, when in these stages of sleep.
To tell the truth, all the previous stages of sleep can hardly be called "very interesting", unless you are not a scientist. But the next fifth stage is of real interest. This is the most incredible and mysterious stage of human sleep, because it is the time when people dream. It is called REM-stage (Rapid Eye Movement), because it is characterized by frequent eye movements, increased heart rate and breathing. REM stage usually lasts from 10 minutes up to 1 hour.
All the above said five stages form one cycle of sleep, which lasts 90 -110 minutes. Taking into account that every next cycle usually lasts longer than previous one, a person can complete up to 5 cycles in a typical night sleep.
Finally, it should be mentioned that in spite of the fact that modern science still has far more questions on sleep than answers, this kind of activity is definitely a vitally important part of human existence, and any sleep disorder should draw our attention just like any other symptom of a disease.
Below is the picture, summarizing the facts on sleep stages, their duration and key features.
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