In Greek mythology, Hypnos (Ὕπνος) was the personification of sleep; the Roman equivalent was known as 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.
Hypnos's three sons or brothers represented things that occur in dreams (the Oneiroi). Morpheus, Phobetor and Phantasos appear in the dreams of kings. According to one story, Hypnos lived in a cave underneath a Greek island; through this cave flowed Lethe, the river of forgetfulness.
Endymion, sentenced by Zeus to eternal sleep, received the power to sleep with his eyes open from Hypnos in order to constantly watch his beloved Selene. But according to the poet Licymnius of Chios, Hypnos, in awe of Endymion's beauty, causes him to sleep with his eyes open, so he can fully admire his face.
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.