*chews grace like a wet squeaky toy*

Grace having a loud dad snore is already a popular headcanon here. On Earth, the lighter sleepers of the PHM crew probably hated sharing a hotel room with Grace because he snores loud enough to wake the dead (…I speak from experience here). But after fifty hears of dead silence, Rocky appreciates that he always knows Grace is alive. And on Erid, they have some domesticated purring kitty cat equivalent, and they too do the “nonhuman character purring headcanons” trope because it’s objectively adorable -so they’re delighted to find that the human actually does purr in some sense.

Grace talking in his sleep is probably unnerving at first. It’s physically not possible to do anything at all in your sleep -not unless something is horribly wrong. They probably have folklore about that, ghosts or parasites or other monsters puppeteering their body. Rocky might wonder at first if Grace is possessed by the ghosts of his crewmates -but if so, random nonsense and memories of simpler times don’t sound like the sort of thing a ghost would say. Grace explaining that humans just do this makes it make a lot more sense. He might eventually come to find it adorable, a glimpse into what he’s thinking about.

Oh but Rocky would really hate sleepwalking. Grace does get a bit quirky at night, he tends to wander a bit… what that means is a bit of a roulette wheel. It’s especially excaberated by stress and sleep deprivation, so it happens a lot. Usually it’s silly things like arguing with a broom or trying to “work”. The PHM crew finds it funny and teases him endlessly, even Stratt gets in on it. Rocky wonders why humans don’t also watch each other if they are capable of moving while unconscious. It probably becomes just annoying after a while because Rocky’s supposed to watch him sleep, not babysit him! But every now and then Grace haunts the ship with ghosts of his past and Rocky is genuinely unnerved; sleepwalking is an Eridian horror trope and he’s actually worried for a moment. (Grace understands, we also find it creepy and he once got decked by a startled crewmate.)

Night terrors though, that’s worse. It contradicts everything Rocky had previously learned about managing human sleep. Normally you’re supposed to wake someone from a nightmare… but with night terrors, you shouldn’t intervene except to stop them from hurting themselves. Grace trying to run away from an unseen enemy and not recognizing Rocky, physical contact making it worse instead of comforting, being haunted by the same memory every damn time and there is nothing Rocky can do to help but wait it out. And then it’s over, and Grace doesn’t remember any of it, and Rocky wonders how humans get any damn rest. (Answer: you don’t! This malfunction happens when something is so horrible the memory endlessly repeats itself in your mind.)

Grace being cursed with an inability to rest in peace. Tormented by the ghosts of past, present and future, and cursed in turn to haunt the ship at night (doesn’t particularly matter where). It’s nothing supernatural, only a combination of stress, sleep deprivation, terrible memories and bad luck with the genetic lottery. But strangled by the red string, practically alone in space, you can’t help but wonder. Grace can try not to think too hard about it -but he can’t repress his actual feelings when he isn’t there to stop himself, and on these nights, Rocky finds himself wishing human sleep didn’t have so many failure points and malfunctions.