What Is REM Sleep?

For many decades, sleep researchers have sought to determine which species ‘have’ rapid eye movement (REM) sleep. In doing so, they relied predominantly on a template derived from the expression of REM sleep in the adults of a small number of mammalian species. Here, we argue for a different approach that focuses less on a binary decision about haves and have nots, and more on the diverse expression of REM sleep components over development and across species. By focusing on the components of REM sleep and discouraging continued reliance on a restricted template, we aim to promote a richer and more biologically grounded developmental–comparative approach that spans behavioral, physiological, neural, and ecological domains. Blumberg et al. describe REM sleep during different times of development and across various species, arguing for a more biologically grounded developmental-comparative approach to understanding REM sleep and its components. They make the case that the diversity observed should be embraced and leveraged to better understand this sleep state.