why when you eat a large meal,

 good hormone’ and it’s also a precursor itself: this time to melatonin – the sleep hormone.
This is why when you eat a large meal, you tend to feel full, then happy, then sleepy. Christmas dinner ring any bells?
This is the opposite of the stress response. This is the aforementioned ‘rest and digest’ response.
And this is another cycle that your body goes through constantly: it moves from fight or flight, to rest and digest. You just don’t notice this because in a perfect world, that shift will be subtle and you won’t feel it too much. You just move slightly up and down the spectrum, becoming slightly more alert and focussed and then slightly less so.
Nevertheless though, this constant fluctuation does have an impact on things like your productivity and your mood. And it is also closely tied to the sleep-wake cycle. When you wake up for instance, you are in a fasted state having slept all night: thus you have high cortisol.