I have the exact same problem with my eleven yearold stepson...
first off, what we did was put an alarm outside his door, (which is basically the kitchen) when he decides to get up at night to take the food, the alarm goes off. we get up and see what he is up too. (this has helped alot)
as for punishment. it doesnt really matter, there has been days where his dad has planned something really fun with him for the next day. its an incentive to not steal food during the night.. but for some odd reason he knows hes got something fun going on in the morning. he still steals the food. we have tried making him write sentences, i have removed everything from his room. (because i am the one that has to search it EVERYDAY and hes starting to get good with hiding the wrappers in his toy box and such.) i have even made him do what we call the jackie chan. (from the new karate kid.) and you can do this with anything (leaving a back pack out, shoes out, not filling the toliet paper roll, anything!) is if he hide the wrapper under his bed. i make him take it from under his bed. to the garbage. throw it away. go back in his room. come back out. take the wrapper out of the garbage. put it back under his bed. take it from under his bed. put it in the garbage. go back in his room. repeat. basically until he gets it. the fact that you have to throw garbage away.
i have really big concerns with his health, he eats a brick of cheese, and then wonders why he cant poop the next day. i am afraid he might develop diabetes. along with black mold off the food, and bugs.
needless to say, he still does this behavior no matter what we do.

My daughter used to do the same thing.. The best thing you can do is find non sugar snacks and give her one after she eats her dinner. If she still does it then after you have her pick it up have her vacuum the area where they were and make her sit in time out for 3 min for every wrapper you found, and dont look under her bed until after dinner because then you can also deny her snack that evenimg. She will get the picture. If she is stealing from other people the best thing you can do is find something of hers you know she willnotice if its missing and when she is in school get it and put it where she cant find it. Sometimes its best to show them than just punish them. Good Luck

I have the same problem with my 13 year old stepson he has been dignosed with mental problems and treated but yet he still steals the sad part is he doesnt steal from his dad just me and my family i need help im going insane..and i dont know how much more i can take its hard i can buy him sugar free he wont eat it he goes in my room when im not home and steals i have put my fridge on the back porch with pad locks and it still isnt working