Since your on night shift you might feel hungry during late hours after 12, and that you must have been munching on junk foods so that you are awake which will to increase in fat in the body. First try to get at least 6-7 hours of sleep in the morning. As soon as you come after work try to have some breakfast wait for an hour and then go to sleep . May be in the afternoon when you get up , have proper lunch which will help you not feeling hungry in the later hours.then as soon as you reach office take a light snack of sandwiches and something to drink . Try and avoid sugary concentrates which will increase the calorie intake.During 2 or 3 am, have a fruit bowl or whole fruit which will help you to feel full throughout your work.but do minimum physical activity in your work place , like taking stairs, going for walk in between breaks and try not sit in the same place for not more than half hour .