We usually think of sleep as a period of a shutdown of mental and activities. During this period, our minds and bodies are supposed to restore or renovate themselves. This idea, although not wrong, is not the whole truth. In reality, a night’s sleep is a phenomenon for more complicated than this.
Our sleep has two parts: one part is called non-REM sleep. The brain gets calmer and calmer, and most physiological functions are slowed down. This is what we usually have in mind and expect when we talk about sleep. However, the second part is the mysterious one. The second part is called REM sleep. REM stands for Rapid Eye Movements. In this phase of sleep, there are bursts of rapid eye movements. The brain waves have an appearance very similar to the waking state. The muscles are in a state close to paralysis, and the brain is active. Most, if not all, of our dreams happen in the REM phase. It is still a mystery why our brain switches from the calm non-REM sleep to the active, busy REM sleep.
Usually, the night sleep starts with non-REM sleep, which then turns into REM sleep. Then another non-REM phase starts followed by REM, and sleep continues in this way. So, we see that our sleep has a cyclic nature. Each cycle is composed of one non-REM and one REM phase. Each cycle takes about 90-110 minutes in adults. So, night sleep usually contains six or seven cycles.
Let’s take a look at a cycle: in the night, we go to bed, turn off the light and close our eyes. The beta waves of the waking brain are still there. As we get drowsier and drowsier, these waves turn into Alpha waves and finally, they change into theta waves. Now we are in the N1 stage of the Non-REM sleep. A few minutes later, we see the waves that signal the next phase of Non-REM sleep, N2. About 20 minutes later, the slow waves of N3 will appear. N1 and N2 were light sleep. Now we are in a deep sleep. Awakening us at his stage is not easy. About 30 minutes later, the signs of N2 reappears and stays for about 10 minutes. At this point, the REM phase kicks off. All the changes in brain waves and eye movements appear, and we possibly start to dream. Awakening at this stage is easier than the N3 stage, but not as easy as in N1. REM sleep stays for a couple of minutes, and then the person passes into a quick N1 to start the next cycle.
As the cycles come one after another, some changes take place: The N3 stage gets shorter and shorter; in the second half of the night, we may not see any N3 at all. So, as the night goes on, most of the non-REM sleep will be N2. The REM also gets longer with each cycle. The first cycle, it may be just a couple of minutes, and it could be half an hour or more in the last. During each cycle, short periods of awakening, particularly in the non-REM part of the cycle, may occur. Another interesting thing is that the person usually moves at the time of stage transition.
