Why is my dog hungry at 3am? How do I stop it?

I'm having trouble with my 2 year old chocolate lab. Within the past two months he has taken to waking up at 3am and whining/barking for food. I tell him no, sometimes swat his but no luck. Occasionally he'll go back to sleep for like 10minutes but then he's back at it again. I don't want to give into him but finally after him crying for an hour I gave him a ilttle foo djust so I could go back to sleep. Does anyone know what I can do to stop this? He used to be on a set schedule of eating at 6am and 6pm. I'm trying to get him back to eating at 6am but he's crying for it earlier.