There are different phases of sleep, and the whole process is cyclical. Not once a night do you go through five phases of sleep and, as a rule, wake up several times. Periodic perusal is absolutely normal – it is an integral part of the phases of shallow sleep, a genetic label inherited from prehistoric times, when a person was always on the alert for danger. That is why, when you wake up, you shudder if you hear something that could be interpreted as a threat, no matter how hard you slept; for the same reason, the instinct tells the young mother that her baby is crying.
The first four phases of sleep are called the phases of slow eye movements (MDG), while the fifth – (surprised, surprised!) – phase of fast eye movements (BDG). The names speak for themselves: at stages 1 through 4, the eyes are fixed, and at the fifth they move, but this is far from the most interesting of what is happening.
In one night, you experience four or five different repeating cycles, each lasting from 70 to 120 minutes. The duration of the cycles increases by the end of the night, averaging from 90 to 120 minutes, but you do not necessarily go through each stage for each cycle. For example, during the third or fourth cycle, you can skip the 4th stage – the phase of the deepest sleep – going from the third to the fifth at once – just as you switch from one gear to another in a car. Deep sleep usually falls on the initial cycles; it takes less time or disappears altogether in the latter.