Papyrus barely sleeps, and when he does he doesn’t sleep for very long or very deeply. He always has a ton of energy and a million things he feels like he could be doing instead, so he’s an extremely light sleeper who tosses and turns a lot and will wake up at the slightest thing, and then he can’t get back to sleep so he’ll spring up and get to work on something. In a Handplates context that restlessness is also has an undercurrent of baseless anxiety to it where he never really feels quite safe or secure anywhere and he also tends to have a lot of nightmares which doesn’t help. It’s hard for Papyrus to find a reason to even try to sleep when he could do something cool instead.
Sans instead will sleep for too long, going through phases where he’ll wake up in bed, not really see a point in getting up out of bed, and then drift back to sleep for another eight hours. Left to his own devices, there’s not a lot that he’ll feel like he HAS to get up and do, and if he does have something planned, he may put it off until the next day and sleep some more. He doesn’t move around very much while he’s sleeping, although his dreams don’t tend to be a lot of fun (particularly in a Handplates context), so I don’t know if I’d say he sleeps deeply exactly.
When they happen to sleep in the same place at the same time though, their habits can actually cancel each other out. Sans usually sleeps on top of Papyrus, which weighs him down and makes it so he can’t toss or turn as much and he sleeps more deeply and longer. Even so, Papyrus can’t stay asleep forever, so his natural restlessness makes sure that Sans can’t stay asleep for too long either, since when he wakes up, he forces Sans to wake up with him. If they’re settled well they can sleep pretty peacefully. In a Handplates context sleeping that way feels the most natural to them (and they have no idea why someone else might think it’s weird), and it makes dreamsharing very easy, although they do at times sleep apart. Sans also helps try to make sure Papyrus gets enough sleep by setting up bed routines (like stories) and Papyrus tries to keep Sans from oversleeping by dragging him off to work each day. They can sometimes kick and annoy each other though like anyone does, haha.
I also did two comics about this kind of thing before (of course I did) where Sans falls asleep when Papyrus picks him up cause he feels safe, and one where Papyrus falls asleep when Sans is nearby cause he knows he's safe. LOOK I KNOW WHAT I'M ABOUT OKAY
Sans and Papyrus (c) Undertale: just roll him off your back
(Not that I blame you, I like these too ^__^)
Papyrus feels his sans crawling on his back uwu
The whole thing.
all of it
...I'll see myself out now