More Biology Questions Back to Ask the Experts Irene Tobler is a researcher at the Institute of Pharmacology at the University of Zurich. It is possible that sleep may have evolved from rest to allow more flexibility within this rather rigid rhythm of rest and activity.
Researchers think that sleep arose to allow organisms to conserve and restore their energy.
All mammals and birds sleep, as defined by the typical changes in brain waves associated with sleep. For amphibia and fish, it is possible to make a behavioral definition of sleep-like states, although not many species have been investigated. Particularly in insects, but also in the scorpion and some crustaceans, researchers have identified a rest state that is similar to sleep. This finding indicates that regulatory aspects of sleep may be present in invertebrates as well. The presence or absence of a true sleep state among these creatures remains to be investigated scientifically.