Read Lullaby Manga:
Seohwa finally gets hired at the best mattress company in the country, but its CEO, Cheolsu Gam, turns out to be a sleep demon who hasn’t been able to sleep for 600 years! Upon meeting, he suspects that she’s his human partner - the only person who can make him fall asleep and allow him to return to the demon world. It’s strictly business, but there’s undeniable chemistry between the two of t more.. hem. Will he be able to go back to the demon world, or will her sudden inability to sleep thwart their destiny?