The physical healing of wounds is expedited by sleep, and sleep strengthens the immune system in general. Rats deprived of sleep in experiments show distinctly inferior healing capacities, develop skin lesions, lose body mass, and are unable to maintain a stable body temperature, ultimately dying of sepsis or just “exhaustion”. Sleep-deprived rats have been shown to exhibit substantially fewer leukocytes (white blood cells), the body’s main defence against infection, and sleep-deprived humans show less than half of the protective antibodies after an inoculation jab as compared to people with healthy sleep patterns.
Sleep, especially stage 3 slow-wave sleep, has also been associated with increased levels of growth hormone levels in the body (growth hormone is an important factor in tissue regeneration and repair), although it is not necessarily the case that increased sleep directly leads to increased growth, or vice versa. Athletes, who put their bodies under a lot of stress and physical pressure, spend proportionately more time in slow-wave sleep than the average person, and growing children spend more time in it than older people. Also, hard physical exercise typically causes a moderate rise in deep slow-wave sleep the next night, all of which supports the cell restoration and repair theory.
The metabolic activity during sleep is mainly anabolic (during which new molecules are constructed and built up) rather than catabolic (where molecules are broken down for subsequent re-use), which also supports the idea that growth and restoration of tissues occur during sleep. The theory has the added advantage of being apparently universal across the animal kingdom, even down to simple unicellular life.
It certainly seems intuitively logical that the body’s quiescence during slow-wave sleep is a good opportunity for the body to focus on physical healing and damage repair, while the more active brain wave patterns during REM sleep suggest a restorative function more focused within the brain. Most of our homeostatic processes (the maintenance, stabilization and regulation of the body's internal environment) occur automatically, and many of them take place during sleep, when the pressures and stresses of everyday life are reduced and there is time and opportunity to devote to this kind of stabilization and maintenance work.
On the other hand, whole-body protein synthesis actually decreases during sleep, and it is still not completely clear that more repair work actually occurs during sleep than during waking hours. Also, while the physical benefits of stage 3 non-REM sleep are readily apparent, it is not at all clear what value stage 2 non-REM sleep - which accounts for over 50% of the adult human sleep period, even though it is present only in rudimentary form in sub-primate species - fulfills.