In short, the question of whether animals sleep depends on how sleep is defined. While many animals don’t exhibit sleep as humans do, characterized by periods of inactivity and specific neural changes, some behaviors in certain animals resemble sleep-like states.

For instance, sponges and most cnidarians don’t sleep due to their simpler biology. However, certain nematodes, like C. elegans, enter a sleep-like stage called lethargus during larval-stage transitions, facilitating development and neural changes. Fruit flies (D. melanogaster) also display behavioral quiescence, which may share similarities with sleep seen in nematodes. Insects and gastropods may undergo estivation, a form of dormancy, but it differs from sleep.

Some annelids, like the earthworm Lumbricis terrestris, exhibit lower activity and reduced oxygen consumption during periods of inactivity, indicating potential similarities to sleep. However, defining this behavior as sleep is subjective.

Sleep in vertebrates, including mammals and birds, has both circadian and homeostatic components, with intensity regulated based on prior wakefulness duration. Even in primitive organisms, sleep-like features seem to be present, although identifying the specific restorative aspects remains a challenge.

While sleep-like behaviors have been identified in many invertebrates, more research is needed, particularly in taxa like mollusks. The complexity of defining sleep is exemplified by the difficulties encountered when studying sleep in various organisms, including unicellular organisms like bacteria.