Sleepy Princess in the Demon Castle

Princess Syalis is trying to get a good night's sleep. Long ago in olden times when people and demons lived together in disharmony, a demon king kidnapped a human princess and imprisoned her in his castle. Bereft, the princess's subjects beat their chests in anguish until a hero arises to the rescue. While she waits for her knight in shining armor, the princess decides to wile away the long hours by sleeping, if it weren't for her pesky insomnia. <em class=de-emphasized>(from manga)</em>

Synopsis from Anime News Network