Wound healing has been shown to be affected by sleep. A study conducted by Gumustekin et al. in 2004 shows sleep deprivation hindering the healing of burns on rats.
It has been shown that sleep deprivation affects the immune system. In a study by Zager et al. in 2007, 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. It is now possible to state that “sleep loss impairs immune function and immune challenge alters sleep,” and it has been suggested that mammalian species which invest in longer sleep times are investing in the immune system, as species with the longer sleep times have higher white blood cell counts. Sleep has also been theorized to effectively combat the accumulation of free radicals in the brain, by increasing the efficiency of endogeneous antioxidant mechanisms.
The effect of sleep duration on somatic growth is not completely known. One study by Jenni et al. 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 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 is some supporting evidence of the restorative function of sleep. The sleeping brain has been shown to remove metabolic waste products at a faster rate than during an awake state. While awake, metabolism generates reactive oxygen species, which are damaging to cells. In sleep, metabolic rates decrease and reactive oxygen species generation is reduced allowing restorative processes to take over. It is theorized that sleep helps facilitate the synthesis of molecules that help repair and protect the brain from these harmful elements generated during waking. The metabolic phase during sleep is anabolic; anabolic hormones such as growth hormones (as mentioned above) are secreted preferentially during sleep. The duration of sleep among species is, broadly speaking, inversely related to animal size and directly related to basal metabolic rate. Rats, which have a high basal metabolic rate, sleep for up to 14 hours a day, whereas elephants and giraffes, which have lower BMRs, sleep only 3–4 hours per day.
Energy conservation could as well have been accomplished by resting quiescent without shutting off the organism from the environment, potentially a dangerous situation. A sedentary nonsleeping animal is more likely to survive predators, while still preserving energy. Sleep, therefore, seems to serve another purpose, or other purposes, than simply conserving energy; for example, hibernating animals waking up from hibernation go into rebound sleep because of lack of sleep during the hibernation period. They are definitely well-rested and are conserving energy during hibernation, but need sleep for something else. Rats kept awake indefinitely develop skin lesions, hyperphagia, loss of body mass, hypothermia, and, eventually, fatal sepsis