Great question, and one I deal with a lot as I am in grad school. I study theatre, so when I'm not reading theory I'm on my feet in class. Plus exercise. So I am mentally and physically active. Here's what I know/have found out.

1. The brain requires glucose to function. If feeling brain fatigue, it actually might be a good idea to eat some fruit, a piece of chocolate, or even starchy carbs to help keep the brain powered. In my experience, VLC (very low carb) is not always a good idea for a student.

2. Getting proper nutrients is vital if you want to keep energy up. I have found things like full-fat Greek yogurt (I recommend the Greek Gods brand myself) to be very filling and a good source of some nutrition. That said, leafy vegetables are a necessity, and tubers like sweet potatoes can help keep things balanced.

3. Fish, or fish oil. It keeps me feeling great, and my brain sharp. Fish is a good source of a lot of nutrients, and omega-3s, which are great for mood and energy. In my experience, it isn't just nutrition to be alert that matters, it's also important to be in a good mood so that the studying isn't as grueling. Nutrition for mood therefore, is part of the equation.

4. Listen to cravings. If you crave meat like beef (which I eat a lot of), then go for it. The body has a knack for telling us what we need.

5. Don't be afraid to snack while studying. Nuts, coconut, beef jerky, etc. are good snack foods, and may help to keep you alert when you've got a lot of reading to do. Keep the body happy and the mind can focus. Plus this way you can avoid the sleep-inducing massive meals.

Hope this helps. I could say more, but I think you get the idea. Best of luck!

Thinking a bit outside the box: intermittent fasting in combination with highly ketogenic foods like MCT oil or coconut oil is pure food for the brain, which is kind of what jim_bob said.

If my brain is feeling tired, a spoonful of L-Glutamine in water is incredibly helpful as well.

I like the green tea suggestion because it doesn't make you jittery, but if you can throw some MCT oil or grass-fed butter into some really, really high quality coffee and combine with some IF, you're golden.

From my experience, I think most clearly on a semi-empty stomach: from the time I get up to 2pm, I subsist on coffee and a few tbsp of coconut oil. I eat a lighter lunch, usually salad, and save most of my eating for dinner. Most days I'm not even that hungry, so it's not like I'm starving myself until dinner. Anyway, after a large dinner, I feel my energies move toward digestion, so I schedule in an hour of relaxing activities that don't require brain power. Then I'm back on my game :-)

If you avoid carbs in your larger meals then you may avoid the sleepiness too. A big beefy breakfast should leave you feeling comfortably full but if you're not distracted by hunger in the morning then by all means leave that meal til lunch - but literally a big steak with eggs or a nice bit of fish. Then eat your veg in the evening when it's time to be sleeping anyway. What I always found more difficult is simply not getting enough sleep and trying to stay awake under the bright lights, in which case your body will crave sugar and you're probably best off obliging it. Well, that or getting more sleep!

Spend some time around Dave Asprey's site (blog, podcasts)...

http://www.bulletproofexec.com

In particular his posts on coffee have been eye opening for me (in short, quality makes all the difference)

http://www.bulletproofexec.com/what-dr-mercola-didnt-say-about-coffee-brains-and-muscle-video/

http://www.bulletproofexec.com/coffee-5-reasons-you-can-perform-better-10-ways-to-live-longer/

http://www.bulletproofexec.com/how-to-make-your-coffee-bulletproof-and-your-morning-too/

Switch to green tea, I think that is the best and easiest thing to do.

Coffee+b1+sugar

So you could do coffee with milk and a glass of OJ will