Looking at Arthur, you wouldn't assume that he's a light sleeper.
He doesn't notice when Merlin lightly traces his eyelids, lips, collarbones, but of course he does when Merlin tries to leave the bed—to get Arthur's breakfast!
Arthur grabs his wrist and blearily says, "Stay." Of course Merlin is more than happy to do just that.
Or he is, until Arthur wakes up an hour later complaining about how hungry he is and why hasn't Merlin gotten his breakfast yet, he is such an incompetent servant…
And Merlin just smiles, ambles off to collect his clothes then goes to get Arthur's breakfast. Before leaving he sends a bright grin Arthur's way and Arthur unwillingly smiles back.