I just have to share the book of Dr. Bruce Lipton - The Biology of Belief - it speaks of how your thoughts affect your cells through the membrane, which can change what products it excretes or if it stops altogether, therefore a large amount of cortisol can be amassed if one thinks too stressfull thoghts, while dopamine and epinephrine levels can get higher through joyful, positive thinking.

It's all explained very clear, with all the scientific terms being well explained. Really, it's in my top5 all time favourites! Strongly reccomended! :)