Is sleep fundamentally different between mammalian species?

An avenue to investigate the functions of sleep is the comparison of sleep in different species, particularly in closely related ones and in species with extreme specializations. The features which are usually investigated are the occurrence of both sleep stages non-REM sleep and REM sleep, their amount per 24 h, the duration of the non-REM-REM sleep cycle and the daily distribution of sleep relative to the light-dark cycle of the environment. Recently also sleep homeostasis has been included, because it is now well established that mammalian species can compensate for sleep loss both by an increase in sleep duration as well as by intensifying non-REM sleep. The occurrence of EEG slow-wave activity has served as a measure for sleep intensity. The capacity to sleep more intensely enables animals to react more flexibly to sleep loss. The comparison of mammalian species has revealed striking similarities in the way sleep is regulated which indicates common underlying mechanisms.