Why is the first stage of sleep non-rapid eye movement (NREM) sleep, and why does rapid eye movement (REM) not happen during this first stage, although we remain partially awake?
A few years back when I was doing my studies on this subject of SLEEP, I had some insights and this question gives me an opportunity to express them and I would put them in the form of a hypothesis:
In my opinion, the rapid eye movement does not happen during the first stage of sleep because it is not part of the “sleep mechanism” which the brain initiates to induce sleep, rather it is the outcome of the “waking mechanism” which the brain initiates. The human brain initiates the sleep process by secreting some hormones, primary among them are GABA, melatonin and serotonin. The factors governing this secretion are in general the hour of the day, the bodily conditions and to some extent other environmental factors. Now when the sleep process has been initiated, a normal adult human being passes through the stages of NREM1, NREM2 and NREM(3&4) in his sleep cycle.
Now, the brain has initiated a sleep process, so it has a duty to initiate a waking process also. When should it do that? For that, the brain simply follows a biological clock and after approximately 90 minutes of initiating a sleep process(in a normal adult human), it initiates a “waking process” which is marked by the secretion of some other chemicals, primary among them being the neurotransmitter acetylcholine and then allows the other factors to decide the future course.
So, after approximately 90 minutes of the first sleep cycle, there comes a transition phase where both the “sleep mechanism” and the “waking mechanism” of the brain are active, and it is a sort of a duel, a tussle between the hormones and chemicals of the two mechanism to outdo each other, to overpower each other or to gain the upper hand. The outcome of all this struggle is in the form of RAPID EYE MOVEMENT and the stirring of the memory cells of the brain which results in the observation of DREAMS during this period.
Since generally at the end of the first sleep cycle the factors responsible for the “sleep mechanism” are very strong, the hormones responsible for “sleep mechanism” easily overpower the hormones of the “waking mechanism” and therefore after a short duration of REM sleep, another sleep cycle gets initiated.
Now, approximately 90 minutes after the second sleep cycle of NREM sleep, the brain again starts the “waking mechanism” by secreting hormones. By this time, the body has already received more than three hours of sleep, and so the factors for inducing sleep are slightly weakened but are generally strong enough to continue the sleep process. So this time the tug-of-war between the sleep hormones and the waking hormones ensues for a little more duration of time than the earlier one when again there is REM sleep and then again, generally speaking the body of a normal adult human goes into the third sleep cycle of NREM sleep.
So, in this cyclic manner when the factors for the “sleep mechanism” have weakened sufficiently, the “waking mechanism” of the brain overpowers it and the body is brought to the waking state.
Since the REM sleep is caused by the “waking mechanism” of the brain, in the first stage of the sleep there is no question of having REM sleep.
I am not sure if psychologists who are actively engaged in research work in this subject think along these lines, but I think this explanation does satisfactorily answer the question and any researcher on this subject is most welcome to verify this hypothesis.(https://en.m.wikipedia.org/wiki/File:Sleep_Hypnogram.svg) [Hypnogram showing sleep architecture from midnight to 6:30 am, with deep sleep early on. There is more REM (marked red) before waking. (Current hypnograms reflect the recent decision to combine NREM stages 3 and 4 into a single stage 3.)