Well, I don't know if it is purely or mostly psychological, but my ability to sleep well seems to correlate negatively with the amount of coffee I drink after 7 p.m. or so. Still, I agree that there is a certain degree of desensitization. I will try black tea next few times I want to be more awake and then switch back to coffee. I don't know well what the difference is between the respective active ingredients. Still,obliquely , I remember reading a book about Starbucks which mentioned that the water purification systems in many systems just cannot get rid of all the caffeine , so much of it keeps poring into reservoirs.