Sometimes, Sherlock dreams of John. It happens far less often than the nightmares do (those awful nightmares that he can only endure, almost never wake up from), and only when he's actually asleep with John. He suspects it has something to do with hearing John's heartbeat, as this always features heavily in the dreams.
In the dreams, he is with John; he is inside John. Next to John's heart, he thinks, because the drumming of John's heart obscures most of his hearing. He can hear what John hears, but it's always muffled by his heartbeat. He wonders how anyone is able to hear anything over the beating of their own heart; this may require experimentation. He sees through John's eyes, and he feels what John feels. Much like he does when awake, but with far more immediacy and depth. He could and does swim around in the vastness of John's emotions because John is level and steady and warm and comfortable even when he's worried. And John is awfully worried for that week.
Sherlock wishes he could do something to reassure John; I'm right here John, not going anywhere until you put me back. But John doesn't seem to hear him. So instead he watches from the comfortable spot next to John's heart as John sits by Sherlock's own bedside— I bloody well look like death warmed over and what is wrong with my hair?--and layers healing and love over Sherlock's empty body. Sherlock feels how he does it, and wonders if he could do that for John.
Watching John try to make him better by main force is one of the strangest things Sherlock has ever seen. He tries to stamp his foot and remind John I'm right here, hello! But John is aching and full of pain that he won't admit to the doctors and Sherlock supposes it's forgivable that John doesn't hear him, doesn't remember. When John falls asleep, sprawled in a hospital chair that shouldn't feel as comfortable beneath them as it does, Sherlock curls up in his rather cosy little spot and sleeps as well, most of the time. John's exhaustion surrounds him until he can't feel anything else; he supposes this has something to do with being in John's body instead of his own.
In these dreams, these memories he relives at night next to John, Sherlock wonders at his lack of concern. It's really quite comfortable inside John, much more so than he ever thought it would be inside someone else's skin. He isn't bored; he can still see, can still observe and think. He's still himself, even if he is perhaps using John's brain to do his thinking. John's brain is really quite remarkable; maybe Sherlock shouldn't be so hard on him about it. John's brain can feel things that Sherlock's can barely contemplate, which is quite interesting. John can do things that should be impossible, can do even more impossible things like scooping Sherlock out of his own body.
He wonders what would happen if John doesn't put him back. Will his body go on living, ever in a state of limbo without him in it? Will he be able to make John aware of him, curled up there in that cosy little spot between lungs and heart? Will John do his work for him, if Sherlock cannot return to his own body? Sherlock contemplates these mysteries while John sits next to him, holding his hand, waiting for the drugs to wear off and waiting for Sherlock to wake.
Spending a week tucked up next to John's heart while John worries about him and tries to help him means that Sherlock knows with absolute precision exactly how John feels about him; he will never need to ask John how he feels out loud, he will never need to see that reassurance from him to know that the feeling is there.
Sherlock shuts his eyes when John realizes that he won't wake if he isn't put back. They fly open again when he feels John's lips against his own, and he flows like a river back into his own body, only for a second wishing that it wasn't happening, grabbing on to whatever part of John he can and losing hold just as quickly. Then, a brief moment of intense pain, and blessed oblivion.
Sherlock thinks these dreams are probably memories and won't allow himself to accept any more than the probability of it no matter what; he doesn't want to remember the week he spent being kept by John, he doesn't want to contemplate the almost unbearable intimacy of it.