Defining and measuring paradoxical (REM) sleep in animal models of sleep disorders

In mammals including humans, the convention defining rapid-eye-movement (REM) sleep — also called paradoxical sleep — states that it is mainly characterized by a low voltage fast electrocortical rhythms, rapid eye movements and a muscle atonia occasionally accompanied by brief distal twitches. When facing sleep pathologies, all these criteria are not always met and identifying REM sleep becomes thus a challenge. Here we review such criteria and report the adjustments proposed to identify and measure REM sleep in animal models of sleep pathologies such as REM sleep Behaviour Disorder and narcolepsy type 1. We also argue that video-recordings synchronized to multi-parametric polysomnography are mandatory to recognize REM sleep and differentiate it from dissociated states in animal models of sleep disorders.