One of my favourites is "50 Natural Ways to Boost Energy". It has a lot of good suggestions for staying focussed on work. I've been trying to use the ideas when I start to feel sleepy, and I've also come up with a lot of my own.
So, at the moment my top 10 ways of not getting sleepy when studying (in no particular order) are:
1. Stand up, stretch, walk
2. Open a window, cool my room down
3. Energizing breathing
4. Put on some music
5. Shower, wash my face
6. Chew gum
7. Put on an essential oil (I have a few from Neal's Yard that I really like, including one called Energy!)
8. Drink water or tea (I've started drinking coffee which is a bad habit that I want to break)
9. Brush my teeth
10. Make a (reasonable) schedule and try to stick with it
And, of course, sleeping enough at night helps a lot too!
The other thing that's key for me is not to aimlessly spend time on the internet at night. It's when I'm most easily distracted and least focussed, so I often end up wasting a lot of time if I start to go down that route (and that's sleeping time that's getting wasted!)
Do you get sleepy when you study/work for long periods of time at a desk? How do you combat it?