1) Disconnecting briefly and unintentionally and if their character dropping like a sack of potatoes is even acknowledged IC it's usually as them drifting off or spacing out for a minute rather than collapsing like a string-cut marionette, or;
2) Falling asleep for the night, on seating of some kind where they're not slumping unconscious to the floor in a heap, they're laying in their bed or curled up on their sofa.
While this @message can be customized there's good reasons to not do so, and most players won't. It's not really an issue as such but especially with new players trying to RP around someone falling into a heap on the floor in the middle of conversation and everyone else just rolling with it (because it happens so often in situations where it just doesn't make sense to integrate into roleplay), I think something a little milder might make sense.
More complicated but something else that I had thought of was a two-step disconnection. A softer emote (like someone closing their eyes) when they disconnect and once that follows a minute later if they don't reconnect which is them actually falling asleep or passing out.
