What biochemical process makes us feel sleepy after eating lunch?

Sleepy after meal: Food in intestine triggers the Parasympathetic nervous system which tells our body to slow down & focus on digesting the food. Brain cells found in the Hypothalamus in the brain, called Orexin, neurons, produce the protein Orexin which regulates wakefulness. Also, Insulin from the pancreas causes increase Melatonin & Serotonin production, hence drowsiness.