Adam Clarke on Psalms 127:2

It is vain for you to rise up early - There seems to be here an allusion to the daily and nightly watches which Nehemiah instituted. The people are worn out with constant labor and watching; he therefore divided them in such a manner, that they who had worked in the day should rest by night, and that they who worked by night should rest in the day; and thus his beloved, a title of the Jews, the beloved of God, got sleep, due refreshment, and rest. As for Nehemiah and his servants, they never put off their clothes day or night but for washing.