It is known that a rather large time-span of our lives, we are in a dream. If the person deprived of this possibility, it would not last long - will die.
But so far, science has not found the answer to the question of why living things is so necessary sleep. On the one hand, it seems clear: brains and bodies need to rest, and on the other hand - what exactly is required shutdown of consciousness?
Nature has programmed all of the organisms in such a way that they necessarily have to sleep. And in a state similar to sleep, even the simplest flow.
However, the relation between the physiological need for sleep, and the process by the animals and is not installed. For example, some species of bats 20 hours a day are in a sleep state, and the large herbivores can have a great deal, and four hours of rest. Horses do so with just a few minutes nap, and then standing up. A newborn dolphins and whales the first month of life (along with their mothers) literally "never sleeps."
At the beginning of the last century, it was pretty widely believed that during the waking hours the body is able to produce a toxic substance that causes us to fall asleep and to calm the brain could hold a "clean". But reliable confirmation of this "model of sleep" was not received.
But in recent times was extended another theory that is gaining more and more supporters. According to the proposed concepts in it, the brain is constantly going on "packaging" and the processing of memories. And at this stage a huge role is played by the synapses of neurons.
While in the waking state, there is an accumulation of memories, resulting in synapses are strengthened. However, this cycle can take an infinite number of times as in this case, new memories could not be formed.
Therefore, in the phase of REM sleep (the most important for the body), is a weakening of synaptic strength, but the accumulated memories are not lost, so we do not forget the events of the past days and years.