Abstract A view of brain organization and sleep function is presented. Sleep is hypothesized to begin at the neuronal group level. Sleep results in the use and thus maintenance, of synapses that are insufficiently stimulated during wakefulness thereby serving to preserve a constancy of a synaptic superstructure. It is further hypothesized that sleep at the neuronal group level is regulated by the production of substances whose rate of production or catabolism is synaptic use-dependent. If sufficient number of neuronal groups are in a sleep state (also called disjunctive state) then the perception of sleepiness occurs. Coordination of neuronal group sleep results from humoral and neuronal projection systems previously linked to sleep regulation. The theory presented is unique in that it: (a) hypothesizes an organizational level at which sleep occurs; (b) hypothesizes that sleep is neuronal — use-dependent, not wakefulness-dependent; (c) hypothesizes that sleep first occurs in evolution when complex ganglia evolved; and (d) hypothesizes the both non-rapid eye movement sleep (NREMS) and REMS serve the same function of synaptic reorganization. The theory is consistent with past theories of sleep function, yet provides a fundamentally new paradigm for sleep research.