If you are having a procedure or surgery in morning, they want you to have Nothing after midnight(npo) if you are having any sort of sedation. Some people may vomit from anethesia, and if you have eaten or drank anything prior you could vomit....choke, possible aspirate, or worse.ps. if this answer doesn't fit you question..sorry.
In my opinion, it is just a common myth. It is related to the other myth, which states if you eat after 8pm, the food you eat will turn into body fat. You need to note that it is more important that your body still needs food intake to repair itself during the night.