Sleepy Princess in the Demon Castle manga volume 2 features story and art by Kagiji Kumanomata.

While waiting to be rescued, Princess Syalis makes herself at home by commandeering a bathtub, befriending a girl demon and participating in a castle fitness challenge. Her plots to get some shut-eye include stealing ice from Frost Demons to cool off on a hot summer night, attempting to make her own DIY waterbed, toying with magic spells and provoking an enchanted flower into releasing a soporific. And then the nightmares begin...