Sherlock can guess what the subject of John's nightmare had been by the way his friend can't look at him for more than a moment at a time without panic crossing his face and making him turn away. He's already apologised for adding that image to the arsenal of destructive things John's mind could throw at him in a moment of weakness, so he doesn't care to repeat it now. It would only be empty words at that point and neither of them need that.

"Have some water," he tells John, focusing on the hang over instead of the nightmare for the moment. He even steps into the room, uninvited, and opens the bottle of water for him.

"You're in your bedroom at 221B Baker Street," Sherlock explains, since he's not entirely sure if John's gotten his barrings off enough to literally not know where he is. That would be a very concerning side effect of the alcohol indulgence and he'd probably suggest having a CT scan on the off-chance it would be something more serious than an ethanol-induced blackout.