Regaining consciousness after a knock out, brain-damage inducing hit to the head was usually a very slow and groggy process, one that Sam Winchester had unfortunately become quite familiar with over his hunting career and had even perfected. It was necessary, seeing as every human/non-human Tom, Dick and Jerry seemed to possess objects or fists with a powerful magnetic pull towards his head (well, their heads, as Dean had also had his fair share of head-related incapacitation) and sometimes it seemed like every other case had one of both of them out cold. It really was quite the miracle that neither of them had irreparable damage.
The first step back to the land of the living was usually his senses flickering back online and it was usually at this point that the speed of recovery was determined; if there was an immediate threat in the vicinity, or Dean needed him or he needed to come back fast in order to avoid being ripped to shreds, his honed hunting senses would alert him and then jolt him back with a shot of adrenalin, which he'd be able to run on for hours afterwards (although who was he kidding, adrenalin was probably the only reason he stayed upright half the time normally).
Other times, when no threat was imminent, a dull awareness came over him, trickling back as his senses slowly and carefully rebooted themselves. The skin on his back would gradually becoming aware of the soft yet lumpy surface of a mattress beneath him, while his nose would take in the musty, generic smell of the motel room, and his ears listened to cars buzzing lazily along the road outside. It was in this stage that Sam felt most at peace, drifting between the darkness and the light, touching neither one of them but still in clear sight of both. Sometimes, he even dreamed.
The next stage brought a painful heaviness that resonated from his temple right down to his toes, threatening to drag him like a weight back under the surface of the murky dark water and suffocate him. It was here he fought tooth and nail, clawing and grasping at the air in his mind, dragging himself up back into the light. This process could take anything between several minutes and several hours, but in the end the result was the same; he broke through the surface to take a great, gasping breath of air.
The final stage of waking up was just that; the 'waking up' bit. Like the first stage, it usually came about gradually, with his eyes opening, his leaden limbs quietly obeying his commands and his strength returning. The splitting headache was a given.
This final part of the process was sometimes accompanied with a very unflattering view up Dean's worried nose.
…
"Sammy? Sam, can you hear me? You good?"
"…yeah, Dean. Nothing worse than usual."