In Greek mythology, the personification of sleep (Roman equivalent Somnus). His twin was Thanatos ("death"); their mother was the goddess Nyx ("night").

His palace was a dark cave where the sun never shines. At the entrance were a number of poppies and other hypnogogic plants.

His offspring consisted of the things that occur in dreams, the Oneiroi. The four principal of these appear in the dreams of kings: Morpheus, Phobetor, Icelus and Phantasos. According to one story he lived in a cave underneath a Greek island; through this cave flowed Lethe, the river of forgetfulness.

Endymion received the power to sleep with his eyes open from Hypnos, in order to constantly watch his beloved Selene.

In art, Hypnos was portrayed as a naked youthful man, sometimes with a beard, and wings attached to his head. He is sometimes shown as a man asleep on a bed of feathers with black curtains about him. Morpheus is his chief minister and prevents noises from waking him. In Sparta, the image of Hypnos was always put near that of death.