Watson's wellbeing is certainly Sherlock's number one priority. If he thought there was any danger of him falling asleep at his post, he would have said so. But he's used to long sleepless nights, and of the three of them is the one uninjured. He can tell Watson is sleeping more soundly, yet he also knows that she will be even more restored in Ted's arms. And she'll be happier if Ted is happier as well.

And Sherlock cares about Ted. Even if it is the last thing he will admit to.

Joan lies down again, grateful for that help. She's feeling remarkably good right now, but she's still sleepy and still fairly weak, so she'll allow him to help her. For now.