Abstract
This chapter shows that key features of sleep are strongly conserved across phylogenetic boundaries and describes how genetics and environment interact to create a high degree of variability in sleep phase, amount, and homeostasis between species. The probable origin of sleep as a response to daily changes in the environment is consistent with it being an ancient behavior, and the variability in its presentation between species suggests it is under evolutionary pressure. The utility of using model systems with smaller nervous systems and wellcharacterized genetic tools to investigate sleep is demonstrated, with an emphasis on the idea that neuromodulatory states are critical to the generation and regulation of sleep.