Comment by mynameismiek

Genetics play a role. I had 23 and Me sequence my DNA two years ago and one of the things they check for is caffeine tolerance. I don't have it. I can drink a 20oz Mountain Dew for lunch (11:30 AM) and it will keep me awake till at least 10:30 PM if not 11 or 12 or even later, without fail. Some people do not metabolize it. Often, I'll get jitters after a single cup of coffee. I restrict my caffeine intake because of this.