A fascinating but still partly unsolved scientific question is to understand the function/s of sleep. Many hypotheses have been proposed, and in this review I focus on the idea that sleep has a role in supporting brain plasticity, learning and memory. Although the majority of sleep research is concentrated on vertebrates, especially humans and mammals, sleep-like behaviour is also evident in some invertebrates. Though surprisingly there are very few studies that have directly tested if there is a mechanistic link between sleep and plasticity. I argue that a phylogenetic approach gives potential insight into the mechanisms and roles of sleep, by allowing an analysis by comparison of the evolution of sleep and rest/activity cycles in organisms showing radically different lifestyles and brain plans. This review attempts to set the scene for a much more profound analysis of these issues using phylogenetic tools.