Infections and the ensuing fever often induce sleep because cytokines, chemicals produced by the immune system to fight infection, have sleep-inducing properties. Furthermore, the current view of cytokines is that they are predominantly inflammatory mediators. There is increasing evidence that cytokines are expressed and perform functions in the normal brain. Researchers are just beginning to unravel the complex interrelationship between the immune system and the sleep function, but the possibilities are intriguing. First, the theories discussed here and others rest more on speculation than empirical data.
Sleep is one of the most complex biological processes, and it likely has multiple primary functions.
Clearly, we need more basic research to investigate the functions of sleep and the molecular and cellular processes underlying it. Second, in looking at theories of sleep function, some inherent difficulties in testing these hypotheses become evident. First, many theories rest on the assumption that sleep deprivation is equivalent to extirpating an organ to determine the impairments produced. Sleep is a complex process involving virtually all organs and systems of the body. That sleep deprivation results in significant impairments, even death, does not explain the primary function of sleep.