Dean's relationship with sleep has always been of the love/hate variety – long periods of deep loathing followed by torrid short-term affairs. Right now, since he finally crashed out with a full stomach and a weight if not exactly off then less apparent on his shoulders with Sam seemingly on the mend, it's the most awesome thing ever. Definitely beats being the alternative: whatever he was dreaming about fades quickly, and his ribs ache and his mouth tastes like something died in there and he's been wearing the same clothes for far too long, and he really wants to just turn over and go back to sleep but the edge of the bed frame is hard against his knees and his arm's prickly with the beginning of pins-and-needles because he's been lying awkwardly, and...
… and that's Dad's voice outside. Dean can't make out everything John's saying – something about how Sam's as stubborn as a mule sometimes but he's glad to hear Jo thinks he's okay (old tactic, one Dean's supposed to have gotten wise to, the one that leads into 'well, actually...') - but it's Dad and the last 48 hours come flooding back, starting with turning up at the Roadhouse and finding out about this secret hunter community or whatever (and thinking that deception the height of his problems) and ending with standing- and then sitting-watch at the end of Sam's bed, which apparently turned into slumping over backwards and passing out.
He'll beat himself up over that later, though; right now he lacks the energy to do anything much. Sitting up makes his head spin, sends the shotgun clattering off the bed (Dean winces at the noise and grasps sluggishly after it, way too late, before deciding to just let it lie) and if that hasn't woken Sam then the combination of a careful jab at the pile of blankets he's currently masquerading as - “Hey, Sammy. Dad's back. Ge'rrup” - and John's knock (which somehow manages to be demanding even though he's clearly doing his best to hold back the way Jo suggested) ought to do the job.