Certain foods contain an amino acid called tryptophan that causes sleepiness. Carbohydrates make tryptophan more available to the brain, which is why carbohydrate-heavy meals can make you drowsy. Proteins from the food we eat are the building blocks of tryptophan, which is why the best bedtime snack is one that contains both a carbohydrate and protein, such as cereal with milk, peanut butter on toast, or cheese and crackers.
There are a lot of foods that contain tryptophan, and they make you tired. Turkey has a high level of tryptophan, that's why you're so tired after Thanksgiving dinner. Here's a website that has a list of foods high in tryptophan. http://nutritiondata.self.com/foods