Sherlock doesn't dream. He never has. Scientifically he knows that he does, he just forgets. He has tried to remember. More times that he can count. Waking up, grasping at images he knows should be there, but no… Nothing.
Sherlock hates sleeping. He hates that he doesn't dream. He wants to wake up, basking in beautiful images his subconscious has conjured up in his sleep. Want to wake up thinking "what did I just dream?". Wants to wake up crying from a horrifying nightmare. The experiences are avoiding him, and he wants to know, wants to feel what it's like to dream. But he doesn't. Sleep is boring. He stops doing it.
John dreams. Sherlock knows this, not only from the nightmares John has told him about and from the rustling and sobbing Sherlock sometimes hears from his room. Sherlock knows this from the way he observes John. He can tell from the way John acts in the morning, right after he has woken up. Even if John doesn't remembers the dream, Sherlock knows from the way he's holding himself, talking, acting, what the dream was about. If he's quiet, holding himself straighter that usual, Sherlock knows that he dreamt about Afghanistan. If he's acting embarrassed the dream was probably of sexual nature. If he's smiling, Sherlock knows he dreamt about flying, running for hours, or even sometimes swimming, breathing underwater, places where it's quiet and John is larger than himself and can do things he normally can't. And sometimes, when Sherlock can feel the affection radiating off John, he's pretty sure that John dreamt about him. And Sherlock likes that. Likes to think that he has wormed himself into John's subconscious, never to leave, that he will be with John forever. He just wishes that John was a part of himself as well.
Sherlock one day wakens breathing into blond hair and his arm slung over the smaller form of John. The dreams are still eluding him, and he's more irritated than usual about that fact, because he knows that tonight, his dreams were amazing. Even if he can't remember them, he knows that they were filled with John. John. John everywhere. John in his subconscious. John here with him, in the physical sense.
And yet... Sherlock opens his eyes, and silently takes in the form of John, peacefully asleep. Even though he can't remember, he realises that sleeping might not be so boing after all, if he gets to wake up like this.