Abstract Sleep is ubiquitous across the animal kingdom. However, the definition for sleep varies significantly across species. In organisms such as C. elegans, Drosophila, and zebrafish, sleep is classically defined as periods of locomotor quiescence with an increased arousal threshold. In mammals, sleep is further subdivided into rapid eye movement (REM) and non-rapid eye movement (NREM) sleep, but the evolutionary origin of these two sleep states remains unclear.