Sleepy Princess in the Demon Castle.

When young Princess Syalis is kidnapped and imprisoned by the Demon King, she's rather indifferent about her situation. But she quickly comes to a terrible realisation—she can't sleep soundly in such an uncomfortable bed! She soon escapes her cell and sets off in search of ways to improve her sleep quality, leaving the castle's inhabitants, and even the Demon King himself, scrambling to keep the unruly princess out of trouble during her chaotic adventures.