"Not at all. You deserve the rest." Castiel slowly retook his seat, hands dropping into his lap. "I'll stay until Dean returns." Sam already knew that neither of them were going to leave him alone in this hospital, so that shouldn't be any surprise. Dean had told him that they didn't leave family behind, and he was sticking to that.
So that seat would remain occupied by the angel for as long as necessary, to ensure that Sam felt comfortable enough to go to sleep, and Dean returned and was reassured that his brother had been properly looked after while he was gone.
After all, it was the least Castiel could do for family.