Everyone assumes it was St. Bart's, but the truth is Sherlock was falling a long time before that.
He admits he was fortunate: he fell into a privileged family; into chemical addiction without much permanent damage; into Greg Lestrade's affectionate exasperation far enough be pushed into a safer dependence on intellectual stimulation.
Sherlock thought that was perfect, but he was more brittle than he knew and dangerous to be around. As Moriarty closed the net around him he found he'd fallen into a much stickier web—affection, trust, interdependence. Sherlock's time away made him think, gave him dreams. They were terrifying.
John can tell some mornings when they've troubled his sleep; when John, caffeinated and fairly crisp on his way perhaps to work, meets his flatmate in the hall, still rumpled and incoherent. Sherlock doesn't say anything in particular but engineers something between a hug and a collision, not actually involving arms necessarily. If it was a particularly vivid dream he may nose at John's neck. John understands the theory and pats him on the shoulder, says something like "I'll be home around four," or "Get some more sleep, you look terrible," or "I'm still glad you came back."
Sherlock fell, he is still falling, into the infinite warmth and safety of John Watson's heart. It's enough to cushion them both.