What is the purpose of sleep?

Sleep scientists admit that they neither know, nor do they really understand what the fundamental purpose or function of sleep is. Such an extraordinary admission seems to indicate that problems may exist both at the conceptual and the methodological levels. This paper attempts to offer reasoned and logical arguments, explaining the purpose of sleep at the conceptual level. It also draws attention to a fundamental linkage between sleep and fatigue. An insight into the underlying theoretical basis of fatigue is seen as an essential prerequisite for understanding the theoretical basis of sleep.