It depends on what to eat. You need to monitor your blood sugar levels. If the food contains a lot of sugar, simple carbohydrates (bread, pasta, etc.), after a sharp jump in blood sugar, it then drops sharply - hence the feeling of fatigue. After a hearty meal with about 1/4 protein, 1/4 fat and 2/4 vegetables (low glycemic index), you probably won't get tired.
Because the blood from the brain goes to the gastrointestinal tract, trying to be filled with the tasty treats from the ingested food. Because of this, the brain wants to rest and sleep.