How common is tryptophan in our food, what effect does it have on our body?

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 :)

North atlantic current or north atlantic drift is the answer