Why are we sleeping at night?

The journey of science in depth sleepy Kingdom found that during the nights are a number of interesting and regular changes of the nature of sleep, is not limited by the availability of slow and fast forms.
On the one hand, identified several stages of slow wave sleep, in which different looks electroencephalogram. The first stage is a drowsy state, surface sleep; electrical activity of the brain reminds picture of quiet wakefulness with the alpha waves normal frequency of 10 Hz. The second stage is characterized by the emergence of so-called sleep spindles alpha-rhythm - group of alpha waves with naturally increasing and decreasing amplitude. The third and fourth stages are the dominance slow Delta waves; in the third stage of the Delta-sleep has an average depth, and the fourth most profound. Stage these usually go after each other from the first to the fourth growing deeper sleep.
On the other hand, it turned out that during the night there are 4-6 cycles sleepy condition, with approximately 1.5 hours. Each cycle consists of a chain of 4 stages of slow wave sleep and ends quick sleep, the deepest form of trance. Then a deep sleep falls sharply, and there is a transition to a new cycle or for awakening. As we Wake up more often during this transition time, at the end of the period of REM sleep, and we remember your dreams.
So, the dream is like a wheel with 5-spoke (stages of slow and fast sleep), and wheel it several times - usually 4 to 6, i.e. on average, too 5, - turns on during the night. The duration of the phases may vary, on average alpha sleep takes about half the time (12 % drowsiness and 38% stage of sleep spindles), Delta-sleep - quarter (third stage 12 %, the fourth 14 %), fast sleep too quarter. By morning periods of REM sleep is usually extended, in General, their duration varies from 10 to 40 minutes, and all through the night quick sleep reigns about 100 minutes.
Why are we sleeping at night? Circadian rhythm shifts wakefulness and sleep subject to the conditions of our lives. Nature programmed in principle the necessity of such a rotation, but the duration of domination desynchronizing and synchronization mechanisms depends on the specific circumstances, it is not programmed once and for all. Experiences with long stay of a person in deep caves showed that in the absence of the change of day and night day can be extended to 36 or even 48 hours with the elongation periods sleepy and non sleeping state.
The presence in the brain active zones usually does not allow us to sleep during the day. Areas of these three.
First, it fronto-parietal centers that control our muscles. While we are in an active state, this area does not allow the mechanisms of sleep to join and act. However, this happens only in normal conditions. If people did not sleep for several nights, the need for sleep increases so that you can even sleep on my legs. Of military practice known not only a dream sentry on duty and cavalryman in the saddle, but the sleep of a soldier on the March - walks and dreams.
Secondly, excited zone that prevents us to sleep during the day, centres associated with the senses: visual (occipital), auditory (temporal), and so on, They are an important counterweight to sleep. Was known for a patient who from all the senses working there remained only one ear and a small area of skin on the arm. Cost close to his ear with a cotton ball and put into the hands of thick mittens - he fell asleep for a few hours.
Third, to sleep during the day hamper the frontal centres in charge of complex mental activities. Even at night feelings, thoughts might not get to sleep until morning.
It becomes clear, why we fall asleep, in the evening before going to bed. All three zones calm down. We then begin to relax the muscles - calm down fronto-parietal centers. We turn off the TV, the light, radio, say around people: "Good night" or "leave me alone!" (depending on) - excitation stops areas related to the senses. Finally, we postpone until the next day, our thoughts and anxiety. Now the sleep center receives the right to join in the work, and we fall asleep.
There are still some points that promotes falling asleep.
On the one hand, everyone is familiar with a soporific effect even, monotonous stimulus, whether monotonous noise of the rain, measuring wheels or the monotonous voice of the lecturer.
Signals that is addressed to one and the same point of the brain, causing fatigue relevant nerve cells. Last because of certain cortical-subcortical relationship leads to activation of the mechanisms responsible for the development of the sleep state.
On the other hand, have a soporific effect associated with going to sleep circumstances: the familiar room, bed time. According to the law of the conditioned reflex they evoke a dream. Quite a powerful factor of this kind is a warm bed. It is a flat, monotonous irritation extensive skin surface, and the usual signal prior to bedtime. This factor is, therefore, irritant and unconditionally, and the conditioned reflex.
The usual factors so important that sometimes help to fall asleep even where they would interfere with the drifting into sleep. One of our famous pianist told me that when he and his brother had graduated from the Conservatory, they played late into the night. Living in the same apartment new neighbor was for them to knock and ask about the termination of the noise. Young men came to him, began to persuade, told about his difficult position in a month exam, they have only one Royal. People were kind and agreed to wait. Another month boys played music to 2-3 hours a night. Finally came the Day of the exam. They successfully withstood him, came home, closed the instrument and went to bed early. However, the rest of them was broken. In the hour of midnight there was a knock at the door. Entered neighbor: "What do you not play? I now without your music can not sleep!"
People complaining about a bad dream, as a rule, mention that in a new environment them to fall asleep more difficult. Sometimes, to outstay usual hour of going to bed - want to sleep, and sleep difficult.