I think there's component in tea that trigger your digestion system to get you poop...I can't really recall but I guess caffeine in tea do their part as well

and never, never ever skip meals even you are doing IF.

physical activities, at first just get yourself walk more, do more physical activities. then slowly observe your surrounding and get more idea how to do more exercise.

e.g office got stairs - if need to go at other level just use stairs, etc etc

probably other friends at this forum section can suggest about workout schedule for night worker as well..