Gradually try and decrease your sleeping hours..

In the beginning you may need to nap for short periods for 10-20 mins in the afternoon.

As someone said make sure to get some exercise and FRESH AIR..

I used to have the same problem and fatigue due to a blocked nose..

Don't try to eat to stay awake-you'll start binging..

If you're studying 3 odd hours after dinner, just get down and do 20-30 push ups-always works!!!:-)