Rest-activity Cycles, Clusters and the Ontogeny of Sleep.

Sleep is a very widespread, almost ubiquitous phenomenon. Mammals, birds, most reptiles, amphibians, and fishes sleep. Sleep has different characteristics throughout the animal kingdom [1, 2], but common ground can be found in the near-total inactivity, the decreased capacity to react to environmental stimuli, and the cyclic quality of sleep. Even insects and molluscs display prolonged sleep-like periods of inactivity. [ABSTRACT FROM AUTHOR]