This was originally posted on Askreddit but i was referred to here:
If you have seen any war movie(older ones) you'd see the one or two guys with a trumpet playing to the other men. Who woke up the trumpet players?
Probably whoever was on watch before that.
99% of forces through time have had watch standing all night. If there are 8 hours dedicated to sleep everyone might only actually get 6 and 1/4 of the force will be on watch at any time in 2 hour shifts.