This is a pretty simple Fabric and NeoForge mod for Minecraft 1.21.11 and later that highlights the mobs that prevent you from sleeping. To avoid it being exploitative, it'll only highlight the mobs if the message you get would be "Can't sleep now; monsters near by", and it only highlights for 3 seconds. This is a fork of the original Monsters in the Closet mod, which is no longer receiving updates for new versions of Minecraft.

If installed on the client, it'll work in singleplayer and any server. If it's installed on the server, than everyone can see the highlights, even those without the mod.