Why do we sleep?

Remarkably, in spite of the many advances in brain imaging technology and medical research, the question of exactly why we sleep remains unanswered. A simple way of appreciating what sleep does for the body is to compare it to eating: a process essential to nourish, sustain and repair the body. A lack of food provokes undeniable pangs of hunger, while prolonged wakefulness leads to an irresistible descent into sleep. These internal drives compel us to act in the interests of our own survival.
There are several common theories as to why we sleep.
Rejuvenation theory
The idea that sleep is a time when the body repairs and heals itself from deterioration that occurs during waking hours. There is significant support for this theory based on knowledge of restorative processes that occur primarily during sleep, including tissue and muscle repair and the release of growth hormone. There are some genes that only switch on during sleep.
Evolutionary theory
Grounded in the premise that inactivity at night offered protection from nocturnal predators. This became an adaptation, favoured through the process of natural selection. The obvious flaw to the theory is that it’s easier to react to a threat when conscious.
Resource conservation theory
Based on the concept that by spending a third of our lives asleep, we utilise less energy which gives our species a greater chance of survival should resources become scarce. While sleeping, energy metabolism and caloric demand are both reduced, lending credence to this theory. On the other hand, the amount of energy conserved by a night’s sleep is approximately equivalent to the calories in a single hamburger bun (161) so it’s hardly a noteworthy saving.
Brain function theory
Sleep has been linked to changes in the organisation and schema of the brain. There is also a recognised correlation between dreaming and these structural adjustments, sometimes referred to as brain plasticity. Infants spend half of their many sleeping hours in rapid eye movement (REM) sleep, the sleep stage when dreams are most common. The intensity of brain development in early childhood is believed to be related to long hours at rest during these first years. Sleep is also believed to support memory consolidation and learning.