Long before the Trojan War, Hera was angry at Hercules, and she persuaded Hypnos to make Zeus sleep while she tormented the hero. Hypnos did as he was told. When Zeus awoke, he was furious. He searched for Hypnos and finally found him hiding in the arms of his mother, Nyx. Zeus overcame his anger and simply warned Hypnos not to try such a trick again, and Hypnos went unpunished.
During the Trojan War, Hera wanted to distract Zeus from the battle so she could assist the Akhaians, 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 Hephaestus, but she was forced to raise the price when Hypnos reminded her of the only time he had dared cast sleep on Zeus.
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 Pasithea, one of the Graces. He turned himself into a bird and, before Zeus could see him, hid in the top 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 his 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 the sun never shines and River Lethe flowed beside it. Although Hypnos was not one of the most active gods of that time, he was famous for the role he had played in the story of Endymion and Selene, the Titaness of the moon. When Endymion was granted immortality and eternal youth by means of endless sleep, he received the power to sleep with his eyes open by Hypnos so he could constantly watch his beloved Selene. The reason Hypnos granted him this gift was because the god of sleep loved the young king/prince/shepherd and thus could always gaze in his beautiful eyes.
A cabin is built at Camp Half-Blood for Hypnos's children after the Battle of Manhattan when Percy Jackson makes all the Olympian gods and goddesses swear an oath on the River Styx to honor the minor gods and goddesses and to claim their demigod children when their children reach the age of thirteen. Percy hoped this would prevent unclaimed demigods from getting frustrated with the gods and causing another war such as the Second Olympian War.
Annabeth Chase takes Jason Grace, son of 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 as 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, then Annabeth said, "If you ask me, this place is even more dangerous than the Ares cabin. At least with Ares, you can learn where the land mines are." Once Clovis hears Annabeth explain Jason's problem of lost memories, Clovis instantly awakens further and hears the story again. He tells Jason to close his eyes, and Jason is afraid of ending up snoring on one of the bunks forever, but Annabeth nods reassuringly, and he does as Clovis asked. His thoughts become murky, as if he was sinking into a dark lake. When he woke up, Clovis was saying, "-serious, all right." Annabeth says that Jason almost faded, although he does not know if she meant literally. Clovis loves his pillow and hates being away from it, and he fell facedown back to his pillow and started snoring again after asking if he could get room service for diner, once Annabeth told him it was almost dinnertime, and then she and Jason left his cabin.
When Annabeth Chase and Percy Jackson were in the Mansion of Night, Nyx mentioned Hypnos. She told them that Hypnos once put Zeus in a deep sleep. Hypnos then fled to The Mansion of Night in fear of Zeus' wrath. Zeus stopped his pursuit of Hypnos fearing Nyx.
Though not a villain or immoral, Hypnos is far from being the most responsible or honorable god to exist. He has been said to do nothing much but sleep and did not play a role in the last Second Olympian War, implying that he is a lazy god (if not the laziest). He also shows to be very cowardly in the past, having hid himself in his mother's arms to avoid Zeus' anger, and when Hera requested his assistance against Zeus yet again, Hypnos only complies when she swore to give him Pasithea's hand in marriage. He was also described to have used his power immaturely when he was a child, much to the other gods' chagrin.
As Hypnos has not appeared yet, his physical description cannot be assured. However, he has been said to be the twin brother of Thanatos, so he presumably has the same "teakwood" skin, black hair, golden eyes, and dark-purple/blue wings. It's unknown if he has the same physical size and musculature of his brother, though, as Hypnos is said to be quite lazy, but it's been worth to notice that he's a shapeshifter. He is often shown with small wings on his head.
- He possesses the standard powers of a god.
- Flight: Through levitation or his wings Hypnos has the ability to fly.
- Hypnokinesis: As the god of sleep, he has divine authority over sleep. The signature power that allows to him induce sleep, visions and even manipulate dreams in a very similar way that of his son Morpheus, but to a much more powerful degree. Since that, while Morpheus took great effort to make Manhattan sleep (and even then only with Hecate's help), Hypnos has been know to use this power with far more powerful beings, in which even Zeus (the gods' ruler and one of the Big Three) has proved in two different occasions in the myths to be susceptible to Hypnos' power.
- Shapeshifting: Hypnos can change his appearance into anything that he wants to be, and has been known to turn into a small bird in the past.
The Roman form of Hypnos is known as Somnus. While Hypnos is the god of sleep and did not 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.
- Somnolence, which is a state of near-sleep or desire for sleep, is named after his Roman counterpart Somnus. Also, somnambulation means sleepwalking (ambulant means having walked in Latin).
- Disorders like hypersomnia and insomnia are named after his Roman counterpart Somnus.
- The words hypnotize and hypnosis comes from the god's Greek name.
- Even though his twin brother has dark skin and black hair, is possible that he not, as both of his children that are demonstrated in the series have light hair and light skin (and in some mosaics, he has blond hair).