Likely you're either eating the wrong thing or you're eating too much of the wrong thing or you're not eating in the proper way (ie not chewing enough).
If you put something in your digestive system that is hard to digest, then your body's systemic energy must be redirected to the digestive system...that in turn starts to divert energy away from your other systems...so you start feeling sleepy.
When you start getting older like me, you start to look into this stuff more seriously!
I really recommend to everyone to throw "green smoothie" into YouTube (no I'm not selling anything here
