Where John needed rest, a chance to be calm, to let his brain sleep, Sherlock needed stimulation, something for his brain to grab on to like a dog with a chew toy (quite literally in fact)...
If his brain had nothing to do, it would spin in circles until it burnt itself out, short circuited, like how a stomach with nothing to eat will digest itself (he couldn't recall if that was true, or if he wanted to look into that? TOO MUCH).
Oh, how he envied John, who'd fallen asleep on the couch, so peaceful. It was hateful. His brain just shut off and everything was well and good in his world. No racing tearing thoughts that threatened to rip him to pieces if he didn't pay attention to them at that exact moment, all at once.
But the drugs... oh the drugs... they silenced everything. Everything. God, it was the only time in his life he'd been able to stop and breathe. As long as he could remember, his brain had always been like this, tearing itself apart. And then it just stopped. And it was fantastic.