Safety
Safety was another thing that prevented his sleep.
John wouldn't lie – he and Sherlock quite obviously shared a dangerous profession; solving crimes and investigating murders that the Yard just couldn't do on their own. It made for many different enemies in many different places.
Sometimes John felt that it would be better to stay up – better to be sure.
Sherlock executed his customary post-case crash and John diligently stayed awake. He sat in the living room and watched the telly with no sound, other times he settled for simply sitting barefoot in the hall and staying on his guard. There was always the undercurrent of paranoia regarding the possibility of someone sneaking in. It wasn't like it hadn't happened before.
They'd likely be after Sherlock, seeing as John himself wasn't regularly made out to be much of a threat. And when Sherlock slept, close to nothing could wake him. One of his extremely few moments of vulnerability. One that he was likely unaware of. What would they do to Sherlock?
Threaten him? Blackmail him? Hurt him? Kidnap him? Torture him?
Murder him?
John was hyperaware of these possibilities. All the time.
Whenever Sherlock insisted on leaving in the early morning hours, in the middle of a case, John was there. Whenever he wandered off, momentarily unaware of his surroundings, John would be there. Whenever he did something that was more than a bit not good, John would be there. Always.
Given his past, he shouldn't have been affected. But he was.
And so, during Sherlock's moment of vulnerability, John would watch. He would wait. And when Sherlock woke up, John would smile and make morning tea like nothing ever happened.
Because Sherlock's safety overrode his health.
Because it always would.