Hypnos was a primordial deity in Greek mythology, the personification of sleep. He lived in a cave next to his twin brother, Thanatos, in the underworld, where no light was cast by the sun or the moon; the earth in front of the cave was full of poppies and other sleep-inducing plants. The river Lethe (the river of forgetfulness) flowed through the cave. He was the son of Nyx (night) and Erebus (darkness), while his wife, Pasithea, was one of the youngest of the Graces and was given to him by Hera. Hypnos and Pasithea had a number of sons called the Oneiroi (the dreams), who according to some sources were three in number; Morpheus, Phobetor and Phantasos.
Hypnos managed to put Zeus to sleep twice, when he was asked by Hera. The first time, Hera devised a plan to avenge the ransacking of Troy by Heracles, Zeus' son; so, Hypnos put Zeus to sleep and Hera unleashed angry winds on the oceans while Heracles was sailing home from Troy. When Zeus awoke, he was infuriated and tried to find Hypnos, who managed to hide with his mother, Nyx.
The second time, Hypnos was reluctant to trick Zeus again, afraid of his wrath. Hera, however, told him that she would give him Pasithea, one of the youngest Charites (Graces), for his wife. After Hypnos made Hera swear an oath by the river Styx that she would fulfill her part of the bargain, he agreed to help her. Hera, dressed beautifully and having a charm that Aphrodite had given to her, went to Zeus, and lied to him, saying that her parents were quarreling, and that she wanted his approval to go and stop them. Zeus agreed, but he was so enchanted by her beauty that he took her in his embrace; at that moment, Hypnos made Zeus fall asleep. He immediately went to Poseidon to inform him that he could now help the Greeks in the Trojan War. That's how the myth has it that the Greeks won the war; Zeus never realised that he had been tricked once again.
Hypnos Is also called Somnus, Hypnus.