The Monster Who Ate Darkness by Joyce Dunbar, illustrated by Jimmy Liao

From chapters.ca:
Why can’t Jo-Jo go to sleep? He doesn't like the darkness under the bed — a monster might be hiding there. And one is! It's a tiny speck of a monster with a huge appetite for darkness, gobbling it up under the bed, in every nook and cranny, and in the wide world outside, growing bigger with every bite. Soon there is no darkness left anywhere, from the earth to the stars. All the world is light, but the monster still has an empty feeling inside. Only a sleepless boy will help him be fulfilled at last.