When you eat in the middle of the night the calories from that food is usually stored as fat and not digested or used as energy like it is during the day.
Eating too much in the middle of the night can sometimes lead to weight gain in some people especially if you consume higher calories foods.
I wake up at night though and will go to the fridge and have a few slices of pizza that I have leftover or have some cottage, a hot dog etc.
I haven't gained any weight and I always get the urge in the middle of the night to have something to eat.
I get hungry in the middle of the night when I wake up anyway to make my way to the bathroom to pee so before going back to bed I eat something and then go to bed.