[paul.fr]

why is it, after having a large hearty meal you just feel so tied, worn out and lethargic?

[another_someone]

Firstly, having filled up your digestive tracts, they are clearly having a lot of work to do to process their food.

Secondly, it makes environmental sense that if you are hungry, you are looking to create activity to obtain food (e.g. hunt down some prey), but having found your prey (e.g. made your kill, and started digesting it), there is no more reason to remain at the same level of alertness.

When I am hungry, I am sometimes lethargic (but not relaxed), but often hyper, and have a heightened sense of smell; whereas on a full stomach I will be more relaxed, but also less aware of smells.