In Greek mythology, Hypnos (Ancient Greek: Ὕπνος, "sleep") was the personification of sleep; the Roman equivalent was known as Somnus. His twin was Thánatos (Θάνατος, "death"); their mother was the primordial goddess Nyx (Νύξ, "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 dwelling has no door or gate so that he might not be awakened by the creaking of hinges.
Hypnos' 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. He was granted this by Hypnos in order to constantly watch his beloved Selene, according to the poet, Licymnius Chios
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.
The English word "hypnosis" is derived from his name, referring to the fact that when hypnotized, a person is put into a sleep-like state (hypnos "sleep" + -osis "condition"). Additionally, the English word "insomnia" comes from the name of his Latin counterpart, Somnus. (in- "not" + somnus "sleep"), as well as a few less-common words such as "somnolent", meaning sleepy or tending to cause sleep.