February 11, 2013 15:16
At this session, we spend about a third of his life. If we deny this, we get sick. But we still do not know well, why should sleep.
At first glance, the answer is obvious: to the brain and other organs to rest and recover. But why can not we have a rest, staying awake and continue to be on guard? Why we can not restore power, and watch? Mystery …
Sleep — so common phenomenon in nature, that it must be incredibly useful. Even fruit flies and nematodes sometimes fall into a state of inaction, of which face is not very easy, it is a dream property even the simplest animals. But long-term observations and have not given the correlation between sleep and physiological needs of animals. So another mystery is the variety of types of sleep.
For example, some bats sleep 20 hours a day, and large herbivorous mammals — less than four. Horses and did enough to take a nap standing up for a few minutes to one day add up to about three hours. Some newborn dolphins and whales are awake with their mothers throughout the first month.
In short, to find a single, universal function of sleep and failed. "The physiological changes during sleep vary widely among different species, — said Frank Marcos of the University of Pennsylvania (USA). — But they all sleep for the brain. " Therefore, most researchers have focused on this organ. The most obvious sign of sleep is recognized as, relatively speaking, the loss of "consciousness" (or reducing the "consciousness" of some animals). And the lack of sleep leads to cognitive crisis not only in humans but also in rats, fruit flies, and almost all other species studied.
The main part of slow-wave sleep is the phase known as the third stage, or the stage of deep sleep (see chart). Its peculiar characteristic of waves of electrical activity in the brain caused by the synchronized activation of neurons, which occurs about once every second. With her blend other stages — rapid eye movement phase, when the brain recalls waking, and the transitions between them.
It is believed that the dream — this is slow-wave stage, for it was in this period that all that should be done to the body's sleep. It is in this phase of the work of the brain most strongly differs from the waking state. In her early waves are especially large when the need for sleep is maximized. If you do not sleep for a long time, these slow waves will grow until you zaklyuete nose.
Sleep function can be divided into two groups: those associated with the "repair" and "maintenance" of the brain, and others — with the performance of the brain unique to sleep problems.
A hundred years ago it was believed that during waking in us produces a toxin accumulation that we finally can not resist and go to sleep to allow the brain to clear. This substance was found, and a modern version of the same hypothesis states that there is a gradual decrease in the afternoon delivery of large molecules necessary for brain function, including proteins, RNA, and cholesterol. Stocks are replenished just in time for sleep. In animal experiments it was found out that the production of these molecules increases during slow-wave sleep phase. However, according to the skeptics, this is only a correlation, and can not say that our desire for sleep depends on the level of these molecules.
The second group also has a long history. Even Sigmund Freud suggested that all dreams are devoted to execution cherished desire (often disguised), but in science, this hypothesis was not confirmed. However, there is good reason to believe that during sleep the brain is still performs a very important and unique feature: it deals with memory consolidation. Memories are not cut out in stone at the time of the event. At first, they are stored in a kind of RAM, and only after that for some reason seemed important, goes to the ROM.
Experiments on animals and humans show that the strongest memories are formed when the moments of memorization and recall shared dream. For example, during one of the experiments electrodes placed in rat brains showed that during sleep the small group of neurons reproduce the mode of activity, which was first recorded, when the rats were awake and learn.
Since 2003, gaining a new hypothesis that unites both explanations. In the center of attention — the synapses through which neurons communicate. It is known that, when the memories of synapses involved in the process of neurons become stronger. The idea is that in the waking state, we are constantly creating new memories and, therefore, strengthen synapses. But this process can not continue indefinitely: at some point would be achieved maximum energy efficiency and new memories would stop forming. The solution to this problem is slow-wave sleep phase: in the absence of incoming transmissions slowly "firing" neurons gradually reduce the strength of synapses in all directions, while maintaining the relative difference in strength between the synapses and allowing new memories persist (see chart below).
"Synaptic homeostasis hypothesis" has received a lot of evidence. Scan of the human brain suggests that our gray matter requires more energy at the end of the day than at the beginning. Giulio Tononi and Chiara Cirelli, of the University of Viskonskin in Madison (USA), propose a hypothesis, have shown that in rodents and fruit flies strength of synapses increases during wakefulness and decreases during sleep. Moreover, when we examine what is required of a certain part of the brain, it is this region generates more intense slow waves during subsequent sleep.
Despite a lot of attention, which in recent years has this hypothesis, it has not managed to win the hearts of everyone. For example, a neuroscientist Jerry Siegel of the University of California in Los Angeles (USA), is of the opinion that the dream — it's just an adaptive way to save energy, without engaging in feeding and breeding. The animal is a quiet place, hiding in it and saving herself for the future. But differences in the way of sleep among species — a consequence of differences in lifestyle.
But most are still inclined to think that the function of sleep is as complex, however complicated brain. Probably, concise answer to the question of why we sleep, barely able to get.