Stages of sleep
Your brain can be measured by measuring the 'waves' that they emit. These waves differ in frequency.
By sticking electrodes onto a sleeping testperson's head it is possible to measure the brain activity. In this way it has been discovered that sleep exists of different stages.
The onset of sleep
While awake, your brain emits rapid and fast beta-waves. When you close your eyes and relax, alpha waves will gradually appear. Alpha waves are slow and calm. You will maybe toss and turn a bit to find the most comfortable sleeping position. Your eyes will move slowly beneath your closed eye-lids. This is called slow rolling eye movement
Stage 1 - Hypnogogic state
Within minutes you will enter the first stage of sleeping, also known as the Hypnogogic state. This is a kind of 'twilight zone' between waking and sleeping and takes about 5 to 10 minutes. In this stage you are easily awaked by the slightest noise.
Your muscles start to relax, you feel yourself floating and you can hear sounds or see images. These images can be anything from colored dots to intricate psychedelic compositions. Those images are called 'hypnogogic images'. You can also have that sudden feeling you are tripping over something or falling down.
Stage 2 - Light Sleep
Now you are really drifting off to sleep. Your brain emits slow and rythmic theta-waves. This stage can be as short as a few seconds or as long as ten minutes.
The theta-waves will eventually be accompanied by quick, intense outbursts of brain-activity. This is generally regarded as the sign that the real sleep has started. It is also called 'Light Sleep', since you are easily awaked in this stage. The funny thing is that if you'd be awoken, you wouldn't have the feeling that you were asleep at all.
Stages 3 and 4 - Deep Sleep
After about 20 minutes you are really deep asleep. Your body is very relaxed, your heartbeat is slow and regular, as well as your respiratory rate. Your brain emits big and slow delta-waves. Stages 3 and 4 are also called slow wave sleep or delta sleep.
It is very difficult to be awoken from this stage. Would you be awoken you would feel fuzzy and disoriented, and you'd want nothing more than going back to sleep again.
It is believed that in this stage the body carries out most of its repair work and healing. When you are still growing, the growth hormone is secreted in this stage. You will literally grow overnight.
Back to 3 and 2
After stage 4 is completed, you will go back to stage 3 and 2. This has taken 90 to 120 minutes. You have now completed one N-REM-cycle.
The sleep cycle is split up in the N-REM and the REM cycle.
REM means Rapid Eye Movement. This occurs in the REM stage, which follows the N-REM stage.
N-REM simply means Non-REM. So you have the cycle where your eyes move rapidly, and the other cycle where they don't.
Stage 5 - REM Sleep
REM sleep is the most interesting of the sleep stages, because from your brain activity one would say you are wide awake. Your blood pressure rises, your pulse quickens and your eyes start moving rapidly beneath your closed eyelids.
Apart from that, your body is virtually paralyzed. It is believed this is an inbuilt protection to prevent you from acting out your dreams. It is in this stage that most dreaming takes place.
The first REM-period usually lasts for about 10 minutes. After that you can briefly awake before the next sleep cycle starts.
The cycle continues
After REM you enter the N-REM stage again, proceeding into REM - briefly awakening - NREM - REM - and so on, until you (have to) get up.
With every cycle the duration of the REM-sleep increases. In the last two hours of sleep the REM-stage can be as long as 1 hour.
The sleep cycle differs for every person. Usually a complete cycle lasts for about 90 minutes. This comes down to about 4 or 5 cycli a night.
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