Certain foods contain an amino acid call tryptophan that causes sleepiness. Turkey is the most well know food with this amino acid. Popcorn also has a similar affect.
Other foods like angel food cake and pasta can also promote sleep. The trick is to increase your carbohydrate intake. Carbs make tryptophan more available to the brain. This is why you are often drowsy after carbohydrate-heavy meals.
Protein can also promote sleep. This is why the best bedtime snack is one that contains both a carbohydrate and protein, like peanut butter on toast.