It’s a matter of tolerance. I don’t know where I am with genetics and caffeine … but I do know that if I steadily increase my intake on a regular basis over a month or two, I can end up drinking a pot of coffee a day and still be able to sleep on time. It’s probably not healthy but when I do lots of physical labour, especially in the summer, I’ll tend to increase my coffee intake. Then in the winters, I drink less coffee and usually just a cup or two in the morning and during these times, if I happen to drink a cup in the afternoon, I’ll be up late and have trouble sleeping.