The function of sleep remains a central enigma of modern biology, in spite of the obvious importance of sleep for normal physiology and cognition. The zebrafish has emerged as a promising new model for studying sleep, its changes with age, and the impact of sleep alterations on cognitive function. Recent studies of this diurnal vertebrate have provided new insights into the dual role of the pineal hormone melatonin and its receptors, regulating sleep in diurnal vertebrates through both homeostatic and circadian mechanisms. Research in zebrafish has also revealed interactions between melatonin and the hypocretin/orexin system, another important sleep-wake modulator. Future investigations should benefit from the conservation in zebrafish of mechanisms that regulate normal sleep, our extensive knowledge of their molecular biology, the availability of multiple transgenic and mutant phenotypes, and the feasibility of applying sensitive in vivo imaging techniques to record sleep-related neuronal activity in these optically transparent subjects. The established sensitivity of zebrafish to many pharmacological hypnotics should also contribute to the development of new, safe and effective sleep medications.