I just played a bit more and noticed there was already something killing you at night the Grue" that you mentioned I didn't exactly try to stay in the dark. same with the old age that was wrong too but still plausible with number of days survived But how about the sleep mechanic? Since you can sleep at night I thought there can be a chance of spawning something like that of a "Nightmare Wisp" maybe that will wander around and try to wrap around your head and make you lose sanity rapidly if you stand still. You can kill them and get a "Dream fragment" that you can maybe create something like that of a sanity torch or I'm imagining along the lines of A light blue Will o' wisp on a stick that runs out like a normal torch that Provides Above average light radius and Sanity restoration while equipped. Although I'm not too sure about how they'll go about their sanity mechanic so I'm just putting this out there.
I noticed that it counts the days that passes and the players ages I suggest you add an "Expiration date" on each of the characters so that when they've played a significant amount of days and have reached the Expiry date a Grim Reaper will spawn at night and will try to reap your soul for GOOD or maybe some other penalty like loss of knowledge and you will have to re-do your Science Research or something like that Maybe any light source will keep the Reaper at bay meaning you don't have to annoyingly deal with him always only at night probably like that Castlevania curse at night. (He cannot exist on any ground with light) Just think about it, as you stay next to your campfire, your lifeline, your safe haven but just outside that safe ring of light lies the Reaper awaiting eagerly with his scythe. I think it fits well with the game's art and theme.