This chapter describes the use of the roundworm Caenorhabditis elegans and the zebrafish Danio rerio in sleep research. Vertebrate sleep research has traditionally been performed using mammalian model organisms. Like mammalian sleep, zebrafish sleep is controlled by a homeostatic mechanism. Several studies have shown that mammalian sleep/wake neuropharmacology is broadly conserved in zebrafish, supporting the notion that similar mechanisms regulate sleep in zebrafish and mammals. Genetic screens are a powerful approach to discover mechanisms that underlie biological processes. Logistical challenges limit the use of such screens in mammals. The genetic analysis using cavefish/surface fish hybrids suggests that a small number of genes with dominant effects are responsible for sleep loss in cavefish, and that these genes may differ among independent cave populations. Importantly, the cavefish system offers the unmatched opportunity to understand sleep regulation and evolution within the context of a well-defined ecology.