Hypnos

Hypnos (Ύπνος in Ancient Greek) is the god of sleep in the ancient mythology and a son of Nyx and Erebus. He is the minor Olympian god of sleep. His Roman counterpart is Somnus. He has at least three sons at Camp Half-Blood: Clovis and two others. Hypnos is probably where the words 'hypnotism' and 'hypnosis' are derived from. Somnus is where the medical term for sleepwalking; somnambulism is derived from.

History
Hypnos is husband to Aglia, one of the Graces, because he did a very dangerous favor for Hera. During the Trojan War, Hera wanted to distract Zeus from the battle so she could assist the Akhaians (Achaians), who seemed to be losing the war. She wanted Hypnos to cast a spell of sleep on Zeus but he refused. At first Hera offered Hypnos a golden throne crafted by her son Hephaistos (Hephaestus) but she was forced to raise the ante when Hypnos reminded her of the only time he had dared cast sleep on Zeus. Long before the Trojan War, Hera was angry at Herakles (Heracles) and she had persuaded Hypnos to make Zeus sleep while she tormented the hero. When Zeus awoke, he was in a rage. He searched for Hypnos and finally found him hiding in the arms of his mother, Nyx (Night). Zeus overcame his anger and simply warned Hypnos not to try such a trick again, and Hypnos went unpunished.

In preparation for this new deception, Hypnos made Hera swear oaths of her sincerity. He agreed to help her deceive Zeus for the hand of one of the Graces, Aglia (or Pasithea). He turned himself into a bird and, before Zeus could see him, hid in the tops of the trees on Mount Ida. He stayed hidden until Hera had seduced Zeus. When the father of gods was dulled by pleasure and sleep, Hypnos flew to Poseidon and urged him to increase his efforts in helping the Akhaians because Zeus was asleep and unaware of the Earth Shaker’s meddling. Poseidon strode through the ranks of soldiers and urged them on. Finally, his bellowing and screeching roused Zeus from his slumber but, in that short time, the Akhaians had turned the battle back on the Trojans. Hera’s trick had worked. Zeus never found out that Hypnos had betrayed him (again). His palace was a dark cave where sun never shines and river Lethe flowed beside it. Although Hypnos wasn't one of the most active gods of that time still he was famous for his role he played in the story of Endymion and Selene, the former goddess of moon. When, Endymion was sentenced eternal sleep by Zeus, he received the power to sleep with his eyes open by Hypnos so he could constantly watch his beloved Selene and because Hypnos himself was in love with the boy.

"Likymnios of Khios (Licymnius of Chios) says that Hypnos (Sleep) loves Endymion and does not close the eyes of his beloved boy even while he is asleep, but lulls him to rest with eyes wide open so that he may without interruption enjoy the pleasure of gazing at them. His words are ‘And Hypnos (Sleep), rejoicing in the rays of his eyes, would lull the boy to rest with eyes wide open.’"

The Last Olympian
In The Last Olympian, Hypnos was built a cabin at Camp Half-Blood probably after he received his own throne on Olympus when the Battle of Manhattan ended. He gets his own throne after Percy makes all the Olympian gods and goddesses vow to an oath to the River Styx that they should honor and claim their demigod children when their children reach the age of thirteen just so that no other unclaimed demigod gets frustrated with the gods and decide to cause another Great War such as the Second Titan War that ended with Luke's sacrificial death so that Kronos, Titan Lord of Time and possessor of his body, could not end up destroying Mount Olympus and that Olympus could survive.

The Lost Hero
Annabeth takes Jason, son of Zeus's Roman counterpart, Jupiter, to Hypnos' cabin at Camp Half-Blood when she realizes that Jason's memories are lost from his mind and that Clovis, Hypnos' demigod son, has the solution to why his memories are lost. There, Jason feels that he is starting to fall asleep and Annabeth tells him that even though Hypnos is the minor Olympian God of Sleep, even the inside of his cabin makes everyone feel like falling asleep. They find Clovis asleep and Annabeth tries to awaken him, but he didn't hear her at all. So Annabeth says his name louder, making Clovis jump at the loudness of her voice and he gets up, half-awake. She says that Jason almost faded, although he doesn't know if she meant literally. Once Clovis hears Annabeth explain Jason's problem of lost memories, Clovis instantly awakens further and hears the story again.

Personality
Hypnos is said to do nothing much but sleep. He didn't play a role in the last Titan War.

Somnus
The Roman form of Hypnos is known as Somnus. While Hypnos is the god of sleep and didn't do much in Greek mythology, Clovis (a son of Hypnos) explains that Somnus would kill people that fell asleep at their posts, such as Palinurus; the helmsman of Aeneas, a Trojan hero. Clovis uses his father as an example of how the Roman and Greek gods, while still the same could be very different from each other.

Trivia

 * Somnolence, is a state of near-sleep or desire for sleep, is named after his Roman counterpart Somnus.
 * Disorders like hypersomnia and insomnia are named after his Roman counterpart Somnus.