The ancient Greeks invoked sleep by appealing to a god called Hypnos, from whose name the word "hypnotism" was coined many centuries later. The link is quite appropriate, for in ancient Greece, if a person went into a deep sleep or trance situation from which it was difficult to arouse him, it was logically assumed that Hypnos had taken control over that unfortunate individual.

So, even that far back, there was a discernible difference between natural and hypnotic sleep. The Romans had a similar god named Somnus, who was blamed for producing odd forms of sleep; hence, the modern term "somnambulism" is used to signify sleep-walking and also to denote a hypnotic state that resembles it.

In artistic works, he is often represented as a winged god, the twin brother of Thanatos (ancient Greek personification of death) who is similarly represented.