I used to work night shifts for the post office doing data entry where I'd sit doing nothing but enter 5 or 6 digit postcodes into a box for 10 hours so I know a few things about having to force yourself to stay awake at work.

The best thing I'd do was not use the elevator on breaks, instead I'd always use the stairs and even though there was a bathroom on our floor I'd always use the one on the ground floor. Going down 3 flights of stairs and back up not only woke me up it also helped loosen my body up that had become stiff from the sitting down constantly.