After you eat, the stomach breaks down the food so you can digest it; the digestive juices contain hydrochloric acid. Sometimes this acid moves towards your gullet, which is very uncomfortable and can cause heartburn, and it's more likely to happen if you are lying down. So that's one reason.

It's often believed that you will have bad dreams if you eat heavily before bed, especially cheese or spices. Evidence for this is inconclusive, but it may be that an overful stomach, or one irritated by rich food, makes you more likely to wake up and therefore remember your dreams. Either way, a rich late meal certainly won't improve your night's rest.

Another common belief is that if you eat before bed you will put on more weight, but again there isn't much evidence for this. It's true if you're asleep you aren't burning a lot of calories, but you can always do that next day. So overall, it sounds as if eating before bed may make you uncomfortable, and if you have digestive problems even more so, but otherwise is probably not harmful.