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Restoration
Wound healing has been shown to be affected by sleep. A study conducted by Gumustekin et al.[20] in 2004 shows sleep deprivation hindering the healing of burns on rats.

It has also been shown that sleep deprivation affects the immune system and metabolism. In a study by Zager et al in 2007,[21] rats were deprived of sleep for 24 hours. When compared with a control group, the sleep-deprived rats' blood tests indicated a 20% decrease in white blood cell count, a significant change in the immune system.

A study by Bonnet and Arand[22] in 2003 indicates that sleep affects metabolism. Comparing normal human sleepers and sleepers with sleep state misperception insomnia, where patients complain of poor sleep but have normal sleep by electroencephalographic (EEG) criteria, the researchers found significantly greater metabolism values for the normal sleepers.

It has yet to be clearly proven that sleep duration affects somatic growth. One study by Jenni et al[23] in 2007 recorded growth, height and weight, as correlated to parent-reported time-in-bed in 305 children over a period of nine years (age 1-10). It was found that "the variation of sleep duration among children does not seem to have an effect on growth". It has been shown that sleep, more specifically slow-wave sleep (SWS), does affect growth hormone levels in adult men. During eight hours sleep, Van Cauter, Leproult, and Plat[24] found that the men with a high percentage of SWS (average 24%) also had high growth hormone secretion, while subjects with a low percentage of SWS (average 9%) had low growth hormone secretion.

There are multiple arguments supporting the restorative function of sleep. We feel rested after sleeping, and it is natural to assume that this is a basic purpose of sleep. Overall metabolic rate goes down during sleep and certain anabolic hormones such as growth hormones as mentioned above are secreted preferentially during sleep. Sleep among species is, in general,...