Despite the fundamental importance of sleep, its function remains controversial. In recent years sleep research has expanded into non-mammalian model systems, with defined behavioral criteria that distinguish sleep from other quiescent states. Using these criteria, we have found that conditions that cause cellular stress can trigger sleep in the nematode C. elegans, a phenomenon we call stress-induced sleep, or SIS. In this talk I will present what we have learned about the genetic and neural components of SIS as well as our evidence that this sleep is beneficial. Last, I will briefly discuss very recent work in other systems that provides support for our speculation that SIS is part of a deeply conserved quiescence program with relevance to mammalian sleep.