In Roman mythology, Somnus ("sleep") is the personification of sleep. His Greek counterpart is Hypnos. Somnus resided in the underworld. According to Virgil, Somnus was the brother of Death (Mors), and according to Ovid, Somnus had a 'thousand' sons, the Somnia, who appear in dreams 'mimicking many forms'. Ovid named three of the sons of Somnus: Morpheus, who appears in human guise, Icelos / Phobetor, who appears as beasts, and Phantasos, who appears as inanimate objects.

In Greek mythology, Hypnos, also spelled Hypnus, is the personification of sleep; the Roman equivalent is known as Somnus. His name is the origin of the word hypnosis. Pausanias wrote that Hypnos was the dearest friend of the Muses.