I'm a grad student I find it hard to concentrate on my work after exercising (cardio+weights) and I can only do after taking a nap. I would like to know if its healthy to have a nap after the work outs?
Exercise also causes CNS (Central Nervous System) fatigue
In a similar manner that your muscles/joints need to recover your CNS requires recovery as well.
Extended CNS fatigue (ie > week is a key sign of overtraining). If you feel 'fried' get more sleep or take a break for a bit.
Sleep, eating more calories, and reducing external stress should help.
Here's a link for some additional tips to adjust your workouts for additional relief. Basically, it comes down to two things. Either don't do everything to the max all the time and/or or schedule for ample recovery (ie sleep/relaxation).
Don't worry, you are no the only one who had this issue, so did I when I first started out my workout sessions, but now after 2 months of continuous workouts, my brain and body has fine tuned itself to accept the strain during the work outs. In short, you'll get used to it, and you will able to study without naps in the long run. Sleep when ever your body feels so, dont fight against your body because you will only be hurting yourself. Keep working out and stay fit!