For breakfast you eat something high in carbs, like oatmeal. The carbs make you tired.
For lunch you eat something high in fat, like a burger and fries. The high fat is the culprit.
For a snack you eat something sugary, like a candy bar. Sugar makes you tired.
For dinner, you eat something high in protein, like salmon or chicken. The protein keeps you alert.
You typically skip dessert. The sugar keeps you awake.