All land mammals and birds have two types of sleep: rapid eye movement (REM) and non-REM (also called slow-wave sleep). Earlier evidence had suggested that REM sleep—associated with dreaming—is essential for physical and mental well-being and learning, and that a lack of it could even be deadly. But the underlying function of REM sleep has been a mystery since its discovery in 1953. Now researchers reporting in Current Biology on June 7 have a new insight into the function of REM sleep, based on studies of a seemingly unlikely animal: the fur seal.