Anyone got any techniques for increasing stamina during exams?

They say that at uni the exams are 3 hours long. My poor brain stops working after little more than one hour.

Anyone got any techniques for increasing stamina during exams?
I used to sell coffee and tea in Clarmont, California to students who went to one of the six colleges there. For cramming for finals or just taking finals, I would sell quarter pound packets of beans. I would tell them to chew 5 beans until they had paste in their mouths then with two tablespoons of warm water swallow it all. This would keep them alert for about 4 hours. And they wouldn't have to get up in the middle of the night to urinate several times. So during tests they were mentally alert also. They told me that what I came up with was the best idea that actually worked, that they had ever tried. A bunch of them had a t-shirt made up for me showing a face with a large open mouth and a hand throwing a handful of beans towards the mouth. On the back of the shirt it said Koffee King of Claremont. I still have the shirt somewhere. This was back in the 70's. You might want to try this for your test.