A man who obeys while still afraid gets the courage afterward, as a result of having moved.
Fear does not have to leave before a person moves; it only has to be denied a vote. Darkness here removes the audience, and the audience is what would have talked him out of it by noon; acting in the dark while afraid is still acting.
Moving before daylight denies fear the hours it needs to build its argument. Morning found the thing already done, and a man who wakes into a completed act is not the same man who wakes into a pending one, which is why what Yahweh asked of him could keep escalating from an altar to an army. Waiting to feel brave first hands the decision to whoever arrives with the best reason to wait.