This is an important question, because average person spends one-third of the time of their life sleeping. Sleep is so important that deprivation of it can kill a person just like deprivation of food can. Other animals and birds also sleep for long or short duration, so there must be some biological reason making sleep essential. However, we are still in the dark about the exact reason behind sleep. We can cite a few inferences drawn by researchers.
In the evolutionary past the very first organisms on earth were without a nervous system, so they did not have two states like sleep and wakefulness. Their descendants of about four billion year ago for the first time started adjusting their activity and inactivity with the light of day and the dark of night. This gave rise to the biological clock in their body, which in turn made some creatures nocturnal and some diurnal.
In case of diurnal humans the daily cycle set by the biological clock is of twenty-four hours – twelve hours for day and the same for night. Body temperature, blood pressure, heartbeats and many other processes in a human body increase or decrease according to this biological clock. The organ experiencing the highest change is the brain. With arrival of sleep the brain’s night-shift begins and it starts large-scale data-processing. Researchers have studied the brain and the processes taking place in it during night a good deal to understand that at night time sleep comes mainly to be of service to the brain. Another proof connecting sleep with the brain: vegetation don’t have brain, hence there is a no state of sleep for trees.