Birds, fish, reptiles and other mammals have at least one thing in common with us: they sleep. Sleep is a central part of our lives and is clearly crucial. Although sleep has fascinated philosophers, writers and scientists for centuries, it wasn't until the early 1950s that scientific research began in earnest. Since then, sleep science has revealed much about the structure and patterns of sleep. Even so, its origins and functions remain largely mysterious
Strictly speaking, the term "sleep" only applies to animals with complex nervous systems. Nevertheless it is possible to identify sleep-like states in invertebrates that allow us define sleep more broadly. These include cycles of rest and activity, a stereotypical body position, lack of responsiveness and compensatory rest after sleep deprivation. Insects in particular have a state very similar to sleep, as do scorpions and some crustaceans.
Even microorganisms, which lack a nervous system, have daily cycles ...
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