The function of sleep remains a long-standing mystery in neurobiology. The presence of a sleep-like state has recently been demonstrated in the fruit fly, Drosophila melanogaster, meeting the essential behavioral criteria for sleep and also showing pharmacological and molecular correlates of mammalian sleep. This development opens up the possibility of applying genetic analysis to the identification of key molecular components of sleep. A mutant of monoamine metabolism has already been found to affect the homeostatic regulation of sleep-like behavior in the fly. The record of Drosophila in laying the foundations for subsequent studies in mammals argues in favor of the force of this new approach.