Re-examining extreme sleep duration in bats

Bats are quoted as sleeping for up to 20 hours a day, an example of extreme sleep duration amongst mammals. Given that duration is one of the primary metrics featured in comparative studies of sleep, it is important that determinations of species-specific sleep duration are well founded. Here, we summarise the evidence for the characterisation of bats as extreme sleepers and discuss whether it provides a useful representation of the sleep behaviour of Chiroptera. Although there are a wealth of activity data to suggest that the diurnal cycle of bats is dominated by rest, citable evidence that they spend the vast majority of this time asleep is limited to a single species studied under conditions which may not be representative of a natural sleeping environment. Furthermore, extreme sleep durations are not expressed under all conditions or in all bat species. This calls into question the validity of using an extreme sleep duration to represent bats in comparative studies of sleep. We suggest best practices for future sleep studies in Chiroptera that will allow for better characterisations of the sleep behaviour of this order and elucidation of the factors that influence sleep duration.