Caffeine, -EV?

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I drink a lot of Mountain Dew, a whole lot. I hasn't seem to have had any affects on my health and I weigh (155) the same as I did in highschool 10 years ago. However I do have problems focusing often and it is hard to get myself to put in enough playing hours. I always assumed it was because I am just lazy but a friend recently suggested that it may be the Mountain Dew and if not the caffeine is still most likely have a negative affect on my decision making ability and game. Any truth to this?