It's not so much strength that allows Sherlock to lift John. Brute strength is just something he has in limited quantities. It's more the knowledge of how to hold his own weight and the weight of someone else in the most efficient way possible to maximise the effort of his strongest muscles and minimise the effort of his weaker ones. Weight, counterweight, and balance. All it is is the practical application to his knowledge of anatomy and physics.

Sherlock can feel John start to stir even before he lies him down on the sofa. He waits until his friend is safely down before moving back and looking him over. He can tell that John's not pleased to be handled like this. It doesn't matter to him, though. John can be displeased all he wants.

"I was moving you to a more comfortable position," he tells him plainly. If John hadn't woken up, he would have already been going to retrieve John's pillow and blanket for him.