Besides some foods containing tryptophan, some foods take longer to digest than others. Your body may be needing more energy than normal to digest your food thus making you sleepy. This can be a good thing if your trying to lose weight. Most people going from a junk food diet to a healthy diet experience this at first. This is just my two cents, believe it or don't!
it sounds like you might solve this by eating smaller meals more often. you can even out the highs and lows in blood sugar levels that way, and you should start to feel better ( less tired) pretty quickly. this sounds very much like the 2:00 p.m. slump that is so common when people don't eat lunch early enough. after they eat, their blood sugar rises and they get so sleepy that it feels like they won't make it to the end of the day without a nap. good luck, I hope the answer is this simple.