The first night John slept without nightmares after returning from Afghanistan was the night he shot a cabbie to save his new flatmate's life. He didn't even notice at first, he was just so tired out that he slept the whole night through (well, until six o'clock in the morning when he was woken by the harsh tones of a violin flooding the apartment, the first of countless times he would find himself in such a situation, and usually at a much more ungodly hour than six), but for John this was the best night's sleep he'd had in a long time. The deafening sounds of machine gun fire, the crimson red blood stains soaking the sand and all the horrors he'd seen unfold that hadn't stopped replaying on his eyelids since, were suddenly replaced by beautiful, blissful, black nothingness. It was heavenly.
Of course, they never went away completely. But the longer he spent living with Sherlock (and not just in the sense that they slept under the same roof - no, Sherlock seemed to have infiltrated his entire universe until their very existences were intertwined together. Not that John was complaining...well, not that in any way he actually meant, anyway), his terror-inducing dreams became less and less frequent. It was probably out of sheer exhaustion if he were honest, he couldn't be certain, but he did know there was a definite correlation between Sherlock's presence in his life and a good night's sleep.
And, to his credit, Sherlock had finally made the nightmares of Afghanistan go away altogether. Except now every time John closed his eyes, all he could see was his best friend falling and falling and falling. And every night he'd wake up, panting and sweaty, just before Sherlock hit the ground - the heart-crushing, sickening sounds of bones crumbling and a skull shattering ringing in his ears, and the look in his eyes as the last ounce of life left his body all he could see.
And that was the worst of them all.