nighthawk

n. a recurring though that only seems to strike you late at night—an overdue task, a nagging guilt, a looming future—which you sometimes manage to forget for weeks, only to feel it land on your shoulder once again, quietly building a nest.

Nighthawks is a famous painting by Edward Hopper, depicting a lonely corner dinner late at night. In logging, a nighthawk, is a metal ball that slid up and down a riverboat's flagpole, to aid pilots in navigation.