It's very common to gain weight when working in the middle of the night. Not getting enough rest tends to facilitate weight gain. I am on FMLA for 6 weeks and dropped 21 pounds in 3 weeks just from getting enough rest every night. I should be whipped back into shape when I return to work in Dec
One thing that helps is pack a small lunch the night before your shift. Take with you to work and eat several hours into your morning shift. This prevents the temptation of stopping and picking up a large portion of McDonalds, Jack In The Box, etc....