The watchman made it very clear that he was vigilant, day and night. (Remember that this is not an actual watchman, but a visionary experience; therefore, being on duty day and night is possible). The watchman did not miss anything, so there could be no doubt about his message.
8 Then he who saw cried out: “Upon a watchtower I stand, O Lord, continually by day, and at my post I am stationed whole nights.