That's because digesting food also takes some effort for your body. If you eat a big lunch/food that's hard to digest, you will make that infamous noon sleepine

That's because digesting food also takes some effort for your body. If you eat a big lunch/food that's hard to digest, you will make that infamous noon sleepiness just that much worse as your body will have to spend energy dealing with all the food in your belly in addition to keeping you awake.