This is inspired by some particularly gorgeous art by mlcamaro on DeviantArt. You can find it here: mlcamaro (dot) deviantart (dot) com/art/BBC-Sherlock-251481485?q=boost%3Apopular%20sherlock&qo=52
From practise, John supposed he learned more from the back of Sherlock's head and the not-quite-profile than he ever did the man's face. But, he admitted, he always tended to learn with what he was given. And when one was 'given' Sherlock Holmes, one was always a step behind. And being always a step behind and to the left, he had a lot of practise reading Sherlock's turned-away-from-him profile and from-behind view of his jaw.
So he sees when Sherlock's so tired that sheer will and force of nature is keeping him up, his curls so tired they lie limp at the back of his neck. Shoulders sloped forwards as if that will keep the direction of his momentum.
He sees the crimp that appears in his collar when Sherlock looks up to see... See what, John doesn't really know, but it's always for something bigger. Some clue. Some hint. Maybe he's looking for the answers to everything. Maybe he's actually just appreciating the sloppy array of colours on the clouds as the sun sets on London. In either case, it means that Sherlock has slowed down and is ready for rest until the next bought of crazed frenzied adventure.