Eli5 why we feel sleepy after eating food
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It’s called postprandial somnolence, and it’s not perfectly understood, partly because there’s a lot of stuff potentially going on. We know there’s a big shift with eating from the “fight or flight” sympathetic nervous system to the “rest and digest” parasympathetic nervous system, and this shift means overall low energy and a drive to reduce muscle use and send blood to the gut instead. There are probably a bunch of other shifts in the brain that contribute, though. Interestingly, chemicals like histamine and orexin that regulate appetite also strongly regulate sleep (i.e. blocking histamine makes you sleepy and hungry.)
Digesting food takes a lot of energy. Your body literally changes where blood is flowing after you eat to send more to your digestive system. That means there is less blood for other things like your muscles.
Your body tells you to slow down.
Digestion requires huge energy.
It is diverted away from all other bodily systems, making us feel sleepy.
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