There’s a mix of factors, including what you eat, when you eat, etc.
But, basically the proteins in food trigger a release of serotonin in the brain, making you relaxed. While carbohydrates help you to absorb some of those proteins which trigger the serotonin.
There are more factors too, and this is an easy to read article
https://www.sleepfoundation.org/nutrition/why-do-i-get-sleepy-after-eating
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