Benefits of exercise: your study time will be more productive, you will sleep better, your brain will work better, etc.
You don't have time? Yes you do:
20 minute, at-home, vigorous exercise with no equipment needed (other than light handweights if you have them): The Jillian Michaels 30-Day Shred.
Search for it on YouTube. Level 1 can be found here.
(The way you're supposed to do the "shred" program is to do Level 1 every day for 10 days, then do Level 2 every day for 10 days, then do Level 3 every day for 10 days...I'm not necessarily recommending the program
(in fact I prefer not to do it every day), but as stand-alone workouts these are really really efficient).
Post here if you know of any other easy-to-follow at-home workouts that can be done in a half hour or less for those of us with no extra time to spare.