When I started at university I could perform with a fresh mind all of the time -- doing a problem set all weekend without reduced performance.
A couple of other students and myself seemed to be several times faster and have larger working memories than others. I made quick work of my studies. 

Now that I have gotten to the ripe old age of 22, I seem to be in perpetual brain fog, and even coffee does not bring me back to my original abilities.
 This disturbs me because I want to go to graduate school.

What do you professors do (especially those who do mostly research) to keep your brain able to function near peak for 40+ hours a week? Are you simply stupider than you were at 18 but abuse coffee and persist in your research anyway?