Chocolate, and coco in specific doesn't sit well with my stomach for some reason. Use to put a bit of it in my morning oatmeal for flavour, even a 1/4tsp would make my stomach feel funny.
In regards to getting sleepy, i think coco releases dopamine via phenylalanine (an amino acid that is pertinent to the production of dopamine).
Same as I've heard. Also with the sugar almost always used to sweeten the cocoa, the insulin preferentially shunts BCAAs to muscle which leaves a higher ratio of tryptophan: BCAAs to enter the brain resulting in a serotonin increase which will make you sleepy.
As regards the dopamine thing, my extensive experience in neurology and psychiatry have led me to theorize that chocoholism is attributable to depression. For the love of God, man, stop chasing the dragon and put down the Hershey's Bar.