Wild animals are extremely skilled at finding sustenance. It's kind of their job. And campgrounds engender that rare and addictive novelty infrequently found in Homo sapiens society: free lunch. So if you want to have peace of mind while you start to doze off, make sure you haven't left a picnic table covered in leftovers before you hit the hay.
Despite the fact that you only heard faint rustlings in the underbrush nearby before you retired to your tent, once cozied up in your sleeping bag, the native fauna will come out in force to snack on whatever s'more, hotdog or burger paraphernalia you've left out. They'll even be beckoned by the scent of toiletries or toothpaste. Stuffed garbage bags on the premise and snacks in your tent are no-no's, too. The goal is to be as scent-neutral as possible.
Some of these scavengers will be small. Some of them won't be. It depends on where you're camping, but nocturnal visitors could range from a curious raccoon all the way up to a snooping hulk of bear. (They like marshmallows and peanut butter, respectively, in our experience.)
So, if you plan on getting some energy by eating breakfast in the morning, make sure your food is stowed accordingly.