At Night, I Become a Monster (Light Novel) by Yoru Sumino

A haunting new novel by the creator of I Want to Eat Your Pancreas. Every night, Adachi transforms into a nightmarish creature—and every morning, he reverts to human form. When he encounters his ostracized classmate Yano Satsuki in his monstrous state, the two develop a peculiar bond. But daylight brings its own form of terrors. Which is worse, the monster at night, or the cruel realities of the classroom by day? A psychological high school fantasy novel from Yoru Sumino, bestselling author of I Want to Eat Your Pancreas.