A considerable number of hypothetical functions of sleep have been proposed, but none of the hypotheses under active consideration has gained enough experimental support to convince a preponderance of sleep researchers. Some researchers even question whether we should expect to identify one primary function of sleep, given the multitude of physiological processes that are affected by sleep. But we do know that sleep is homeostatically regulated, and we have discovered a great deal about the physiological mechanisms underlying sleep homeostasis. Other homeostatically regulated behaviors have one or a small number of functions, and those functions are closely linked to the homeostatic control mechanisms for those process. We can therefore apply what is already known about sleep homeostasis to test and evaluate hypothesized functions of sleep. It should be possible to trace the connections between the cellular and molecular basis of a hypothesized function and the cellular and molecular mechanisms underlying homeostatic responses to sleep deprivation. Hypothesized functions that do not plausibly admit of such connections can reasonably be rejected. Four current hypotheses suggesting that the function of sleep is to assist in the process of activity-dependent synaptic reorganization are critiqued to demonstrate how what is known about sleep homeostasis can be used to assess hypothesized functions of sleep.