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. That is tantamount to saying that the function of the liver is to prevent the negative effects of liver-deprivation. It is also difficult to know what kinds of evidence would support or refute a hypothesis about sleep function. Ultimately, to understand sleep, we need to understand the functions of sleep. And to understand the brain, we need to understand sleep.