Sleep is a ubiquitous component of animal life, and prolonged sleep deprivation is fatal in both vertebrates and invertebrates. The physiologic function of sleep, however, is not known. We propose here that sleep provides a period of time necessary to reapportion resources within neurons and neural systems that become sub-optimally distributed during active waking. Three specific examples of such reapportionment during sleep are suggested: (1) the return of the neurotransmitter, glutamate, to synaptic vesicles at presynaptic sites most active during waking, (2) the intracellular movement of mitochondria from neuronal processes to the cells soma where mitochondrial replication can occur, and (3) the readjustment of the level and distribution of neurotransmitters within the brainstem modulatory systems and elsewhere that must function in an integrated fashion during waking. Experimental approaches that might be utilized to test these hypotheses are suggested. To cite this article: J.A. Gally, G.M. Edelman, C. R. Biologies 327 (2004).