Sleep: Findings in Invertebrates and Lower Vertebrates

Abstract

Sleep is almost universally present throughout the animal kingdom, yet how sleep has evolved is not known. From an evolutionary aspect, a majority of animals that have been studied so far exhibit sleep or sleep-like states. Studies on sleep in different species belonging to vertebrate and invertebrate categories have provided novel insights into the evolution of sleep. The presence of a sleep-like state in jellyfish, a simple diploblastic animal having a noncephalized brain and no centralized nervous system, suggests that sleep/sleep-like state might have evolved even before brain cephalization occurred. The sleep-like state has been reported through observational studies in several species such as coelenterates, nematodes (roundworm), annelids, arthropods and mollusks, fishes, amphibians, reptiles, birds, and mammals. Electrophysiological correlates have also been recorded in a few species. The changes in low-frequency waves and spike-like activity in the EEG during the sleep-like state have been reported in crayfish, Drosophila, octopuses, some frogs, green iguanas, and box turtles. The activity of some EEG electrical waves changes from active to sleep-like state in all these organisms. This electrical activity could be the electrophysiological signature of the sleep-like state in nonmammalian and nonavian species. It is possible that such electrophysiological correlates evolved phylogenetically in mammalian and avian sleep. In this chapter, we have attempted to provide a comprehensive overview of the current knowledge about the evolution and presence of a standard signature of sleep across different species.