The "slow" sleep
The sleep during these four stadiums are called slow, because the brain waves are slower than they are when you are awake.
When we are awake and active, our brain sends us electrical waves with very low amplitude (low tension) but with high frequency, between thirty and fifty per second. Those are the so called beta-waves.
As soon as we close our eyes and relax, both physically and mentally, the frequency decreases to about ten per second, but the tension increases to about five times more, and these are the famous alpha-waves, which appear when we are awake but passive. This is a sensitive state. To make the person notice something, to make him open his eyes, it is enough to just touch or talk to him. Then the alpha-state gets replaced by the beta-state.
If you do not get disturbed and you stay in bed, the alpha-state remains. Then the brain waves start to get slower, the frequency is four to five per second, and the theta-waves take over. This happens just before we fall asleep, and the theta-waves remain during the first stadium of our sleep. During the first stadium the dreams are very much alike the ones during the REM-period.
Soon the theta-waves are replaced by sigma-waves. This is the second stadium of our sleep, and it is shown by the fact that the waves are getting faster, eight to ten per second. We stay in this stadium for twenty minutes. The dreams during the second stadium are different. Then the dreams' content are the same, only it consist of fewer pictures, seem more like thoughts and are not all that detailed.
After that we enter our third stadium of sleep. The waves get slower, one to three per second, but the tension gets much higher, about twenty-five times higher than when one is awake and activated. This is the delta-waves. The third stadium lasts for about ten minutes.
When more than half of the delta-waves have been printed on the paper, we say the person is in the fourth stadium, the deep sleep. At this point many of your vital functions, such as breathing, heartbeat and blood pressure, lie at their lowest level. This stadium lasts much longer. It ends about ninety minutes after you fell asleep. When it ends, you usually return to the second stadium again.
REM-sleep, the "fast" sleep
Suddenly, the second stadium, that you have returned to, is interrupted by a sensational change. The brain awakens and starts to send out the passive alertness kind of waves, the alpha-waves.
Kind of awake stadium
After the fifth stadium we return to a kind of awake stadium for one to two minutes. This period is too short to be remembered and we fall back to sleep again. This period of one to two minutes is the most vulnerable time of our sleep, it is easy to wake up from sounds etc.
After that a new cycle begins, and so on. It is constructed the same way.
Only the first two cycles include the fourth stadium and during the night the fifth stadium gets longer, twenty-five to thirty minutes instead of ten to fifteen. Throughout the night you dream, totally, about one hundred minutes.
- Sömn - Pierre Fluchaire. Bonnier Fakta Bokförlag AB, 1986, 103-109
- Sömn och sömnrubbningar - Reidun Ursin. Bokförlaget Natur och Kultur 1986, p.31-32