system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why does eating make you sleepy?

I know that's it not a good idea to eat right before you go to sleep, your body digesting the food uses energy and the process disrupts getting that perfect night's sleep. So why the hell does eating a big meal make you sleepy? It seems really counter productive and it's super annoying.
gpt
In addition to the High energy digesting everyone else mentioned, it's important to be aware of Insulin and insulin crashes. This doesn't happen much after small meals/snack, but if you eat a lot of food (especially with a lot of sugar - natural or otherwise) your body gets a massive influx of bloodsugar, which your body needs to distribute throughout the body. This is done with insulin, and if you get a high amount of sugar in the blood your body reacts by making A LOT of insulin.  You usually end up with more insulin than you actually need, so you end up with (relatively) low blood sugar after a bit.  This low blood sugar is accompanied by lethargy, etc and likely contributes to a sleepy feeling after BIG meals.