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.
Somnus and Mors, Sleep and His Half-Brother Death by John William Waterhouse
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.
Hypnos marble head (National Roman Museum, Rome)
Bronze statue of Hypnos (Museum of History & Archaeology, Almedinilla, Spain).
Hypnos and Thanatos carrying the body of Sarpedon from the battlefield of Troy; detail from an Attic white-ground lekythos, ca. 440 BC.
Ariadne asleep at Hypnos's side. Detail of ancient fresco in Pompeii