I can't give you an exact number or anything for how common it is. However, I can say that it is QUITE common in our food.
There's many effects it has to the human body, therefore I'll only list a few.
~social behavior~irritability~mood swings~sleepiness
Also, if tryptophan is used as a drug it can be used to treat depression. Though, it can only help to a certain extent. It varies as well; depending on the person. (Don't quote me on that completely though.)
I hope this helped :)
