Inspired by a comment on the TV Tropes Headscratchers/Tangled page. If you want to read it, search "This does raise an interesting question" on that page, preferably after finishing this story. WARNING: TV Tropes is a whirling black hole vortex of a time suck. Enter at your own risk.
This the story of how Princess Violet, lover of light and heir apparent to the throne of the sun kingdom, began singing in her sleep.
Violet ran down the hallway and then stood trembling at the half-open door to her parent's room.
"Mama?"
"Yes, sunshine?" Queen Rapunzel replied. She lit the nearest lamp. (Flynn snored quietly, quite the heavy sleeper when he had a comfortable bed.) The queen crossed the room and pulled the youngest of the family into a gentle embrace. "Did something wake you?"
Violet shook her head, nestling into the warmth of her Mama's sleeping gown and the safe light of the queen's bedside lamp. "I couldn't sleep. Too dark; it's scary."
After a scare that nearly burned the young princess' bed with her in it, no one in the castle would allow any sort of lamp in Violet's room. The girl would open them up to see the fire inside, as fascinated with lanterns as her mother ever had been. Even during the festival—now to celebrate the long-lost princess' return—Violet was allowed to handle lanterns only under close supervision.
Rapunzel brushed her fingers through her daughter's close-cropped, brown hair—"Like Mommy's!"—as she sighed through quirked lips. If the poor thing hadn't inherited her mother's love of the lanterns, she could have had one in her room. But then the princess probably would not have inherited Rapunzel's need for light, either.
"The stars aren't enough?" Her window had been directed right at her favorite constellation. There was already a name for it, but she just called it The Protector.
Violet shook her head against her mother's skirts and then looked up. "It's too cloudy. And there's no moon."
Rapunzel just held her daughter for a moment, then nodded. "Come here. I want to show you something." The queen took the princess' hand and led her to the balcony garden. Everyone in the castle understood that this place was Rapunzel's own, and Violet knew that if Mama was taking her here, it was for something important.
Rapunzel hesitated to give away a part of what she had from the first eighteen years of her life. She hadn't wanted Gothel's things, but her books (worn, sometimes hard to read), her paintbrushes (now nearly impossible to use), her hairbrush, and…
"Do you remember how they took a flower to save Grandma's life?"
Violet nodded, not sure how this was going to help the fact that her room was darker than any paint she'd seen her mother use. The girl shivered.
"Well, they did take the flower, to make a tea and make her better. That's why I'm here." Violet grabbed her mother's skirts perhaps a little tighter than necessary. "I'm not going anywhere," Rapunzel soothed.
"And the flower didn't move far, either. They brought a gardener along, to make sure the flower stayed alive on the trip back from wherever in the kingdom the flower might be. The stem was alive, and kept in the Royal Gardens. When I came home, the gardener gave the flower to me as a birthday present, or a homecoming gift." The princess' mother picked up a plant in a very small pot and set it in her daughter's hands.
"Do you remember the song?" Rapunzel whispered.
Violet nodded. "Flower, gleam and grow—" was as far as she went before she almost dropped the plant. "It glows!"
Her mother grinned. "Yes. The flower glows if you sing anything, though shines brightest for that song."
"Really?" The girl made up a tune. "La, la, la…oh, I love it! Can I keep it, Mama?" Violet beamed up at her mother, glowing brighter than the flower.
Any hesitation Rapunzel had felt fell away in the face of her little girl's smile. "Of course you may. It's your new light, for when the nights get too dark. Care for it well; it's almost as special as you." Rapunzel swept Violet up in her arms and rubbed noses. "Now let's get you back to bed, sunflower."