If I have lot's of filling food, I feel tired and can't concentrate and want to sleep.
But if I don't eat much except (for example) a few cookies, and perhaps only have a large (but not very large) meal late at night, I feel a whole lot better.
I can certainly concentrate better, and I'm usually happier.
Foods with a lot of fat content will generally make you feel more lethargic. I have read that kepin a log of what you eat and how you feel for a period of two weeks will help you equate what foods work best with your metabolism.
Additionally, make sure you take a good multivitamin every day. If you feel tired, adding an additional B complex vitamin may help as well.
Also, stress of any kind, regardless of what type of foods you are eating could contributed to fatigue, insomnia, etc.
-Miked
