Believe it or not even humans and many other life forms spend a significant fraction of their life sleeping, no one really knows why!!! Your question has no unequivocal answer!!! There are some ideas, however. One is that during sleep the brain is processing, compiling, 'sorting out' and storing in brain memory cells the events of your previous 'awake' state. To some extent sleep or rest may be required for your physical body to repair itself from the daily grind.

In sleep, the motor controls of your arms, hands, legs, etc., is "turned off" so that when u dream of running your legs are not flopping about!! So, in that sense there is a cut off between the brain and the rest of you. Here are some further references that will take you into the truly fascinating areas of neurobiology and many other areas of knowledge.

An article on SLEEP IS PASTED BELOW FROM WEB SITET. Allison and D. V. Cicchetti. (1976). Sleep in mammals: Ecological and constitutional correlates Science 194: 732-734. (PubMed)T. H. Allison and H. Van Twyver. (1970). The evolution of sleep Natural History 79: 56-65.

Why Do Humans and Many Other Animals Sleep?

To feel rested and refreshed upon awaking, most adults require 7-8 hours of sleep, although this number varies among individuals (Figure 28.1A). As a result, a substantial fraction of our lives is spent in this mysterious state. For infants, the requirement is much higher (about 16 hours a day), and teenagers need on average about 9 hours of sleep. As people age, they tend to sleep more lightly and for shorter times, although often needing about the same amount of sleep as in early adulthood (Figure 28.1B). Getting too little sleep creates a "sleep debt" that must be repaid in the following days. In the meantime, judgment, reaction time, and other functions are impaired. Drivers who fall asleep at the wheel are estimated to cause some 56,000 traffic accidents annually and 1,500 highway deaths.

Sleep (or at least a physiological period of quiescence) is a highly conserved behavior that occurs in animals ranging from fruit flies to humans (Box A). This prevalence not withstanding, why we sleep is not well understood. Since animals are particularly vulnerable while sleeping, there must be advantages that outweigh this considerable disadvantage. Shakespeare characterized sleep as "nature's soft nurse," noting the restorative nature of sleep. From a perspective of energy conservation, one function of sleep is to replenish brain glycogen levels, which fall during the waking hours. In keeping with this idea, humans and many other animals sleep at night. Since it is generally colder at night, more energy would have to be expended to keep warm were we nocturnally active.

Furthermore, body temperature has a 24-hour cycle, reaching a minimum at night and thus reducing heat loss. As might be expected, human metabolism measured by oxygen consumption decreases during sleep.

Whatever the reasons for sleeping, in mammals sleep is evidently necessary for survival. For instance, rats completely deprived of sleep die in a few weeks (Figure 28.2). Sleep-deprived rats lose weight despite increasing food intake, and progressively fail to regulate body temperature. They also develop infections, suggesting an impairment of the immune system.

In humans, lack of sleep leads to impaired memory and reduced cognitive abilities, and, if the deprivation persists, mood swings and even hallucinations. The longest documented period of voluntary sleeplessness is 264 hours (approximately 11 days), a record achieved without any pharmacological stimulation. The young man involved recovered after a few days, during which he slept only somewhat more than normal, and seemed none the worse for wear.

"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express in numbers, your knowledge is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of Science, whatever the matter may be."
Lord Kelvin ( 1824-1907)

Dreaming is thought to be the response to the lower brain generating apparently random signals through to the rest of the nervous system, stimulating it. I do not believe that it is not known why the lower brain does this, only its effects.

There are two ways that I can answer this question. One answer relates to the causal processes in the brain that lead to dreams. The other is to describe the functional role of dreams.

The first can be summed up by saying that dreams result from neural impulses during sleep. I should point out that there is no consensus on this neurological explanation. Most explanations also implicate rapid eye movements (REM, which occur during deep sleep).

Waking conscious life is full activity to keep us busy. However, we are undisturbed (and uninhibited) during sleep. For this reason, dreams probably function as an alternate consciousness that should be considered just as real as waking consciousness.