Sleep in amphibians and reptiles: a review and a preliminary analysis of evolutionary patterns

Despite the ubiquitous nature of sleep, its functions remain a mystery. In an attempt to address this, many researchers have studied behavioural and electrophysiological phenomena associated with sleep in a diversity of animals. The great majority of vertebrates and invertebrates display a phase of immobility that could be considered as a sort of sleep. Terrestrial mammals and birds, both homeotherms, show two sleep states with distinct behavioural and electrophysiological features. However, whether these features have evolved independently in each clade or were inherited from a common ancestor remains unknown. Unfortunately, amphibians and reptiles, key taxa in understanding the evolution of sleep given their position at the base of the tetrapod and amniote tree, respectively, remain poorly studied in the context of sleep. This review presents an overview of what is known about sleep in amphibians and reptiles and uses the existing data to provide a preliminary analysis of the evolution of behavioural and electrophysiological features of sleep in amphibians and reptiles. We also discuss the problems associated with analysing existing data, as well as the difficulty in inferring homologies of sleep stages based on limited data in the context of an essentially mammalian-centric definition of sleep. Finally, we highlight the importance of developing comparative approaches to sleep research that may benefit from the great diversity of species with different ecologies and morphologies in order to understand the evolution and functions of sleep.