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.
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.
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.
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.