eLife assessment
Drosophila is a powerful model organism for understanding the molecular and neural regulation of sleep. However, methodological limitations exist that would appear to limit the relevance of work done in the fly to our understanding of mammalian sleep. In this important work, the authors provide physiological, behavioral, and molecular evidence for the existence of two potential sleep stages in Drosophila. The experiments are generally well conducted and the authors' interpretations of their results are solid overall. Although technically innovative and conceptually provocative, there are aspects of the approaches used and results obtained that leave the central conclusions open to interpretation.