The myth of the Sandman if he was from Greek mythology instead of Germanic folklore:
Long ago, there lived a woman named Oneira, who was a talented and imaginative storyteller. Her stories would never fail to put any child to a deep and pleasant sleep.
Oneira's talent drew the envy and ire of Calliope, the muse of epic poetry. And so Calliope schemed to bring her down.
At first she called on Aletheia, the goddess of truth, and told her of Oneira's "lies and fabrications". But when Aletheia came to confront Oneira, Oneira told her a story so beautiful that Aletheia couldn't help to see the profound truths it conveyed, despite being fiction.
Frustrated, next Calliope tricked Oneira into drinking from a pond which was the favorite hunting spot of the goddess of the hunt, Artemis. Upset, Artemis asked her brother Apollo to steal Oneira's voice, so she won't be able to tell her stories.
After the deed was done, Apolli realized he was used. He made it up to Oneira by teaching her how to write, so that she may still express her imagination to the masses.
Foiled once more, Calliope tried one last scheme. She approached a mortal man named Amathos. He was a skilled orator with a beautiful voice, but lacked imagination. She told him of Oneira's plight, and suggested that he should help her by reading Oneira's tales aloud. Amathos took off to meet Oneira, and the two quickly became collaborators and later lovers. They became famous throughout the land for their performances. And Amathos' voice was so powerful it reached Olympus.
One night, Calliope cursed Amathos with insomnia. As he woke, she told him that he owed her a debt, and all he has to do to repay it is get Oneira to write a specific story. A story about a world where the gods did not exist. Oneira was suspicious when her unimaginative husband told her he had an idea for a story. But out of love, she agreed to write it.
Once the story was ready, it was time to perform it before a crowd. And when Amathos read it aloud, the god Zeus heard it and became mad, thinking that the two lovers were trying to turn humanity against him. He went down to the Earth and cursed Oneira and Amathos.
Oneira was turned to sand, and Amathos was punished by having to carry her in a large sack on his back for all eternity.
Now every night, Amathos, still stricken with insomnia, must place a single grain of Oneira into each eye of every child, putting them to sleep. Only when the sack is empty, will Oneira's soul be free. But every time Amathos himself gives in to exhaustion and falls asleep, the pieces of Oneira magically return to the sack, ensuring that his task is never complete.
The sand, still carrying the pieces of Oneira's soul, continue to imagine fantastical stories and show them to the children in their sleep. This is what we call dreams.
And that's why you have some hard gunk in the corner of your eyes when you wake up.