During my first 2 years, I loved to workout right after classes.
After 4-6 hours of being talked at, my brain was about fried as was my attention span. Working out and clearing my head of all things school related was a welcome break. Then when it was time to study, I was relaxed and able to focus.
If you're an extreme gunner, you can get lecture MP3's on your MP3 player and listen to those as you run, or have notes with you in the gym on the treadmill or ellipse machine. I wasn't that much of a gunner and needed some time to relax and listen to music and remember that there is more to life than just school!
I didn't work out as much my first semester, tried to study all the time and ended up falling asleep on my notes constantly and didn't get great grades.
My 2nd semester I started running and working out daily. I found that I had more energy, I was able to focus better, stay
awake while studying and my grades improved drastically! Summer before 2nd year, I got married and my wife is an exercise freak! I started working out and running even more than I had before and my grades got even better. Add that onto weighing 20lbs less than I did when I graduated college and I can't think of a good reason not to workout during med school!
Besides, research has shown that cardiovascular exercise can help prevent Alzheimer's and other dementias!