I would recommend you don't make caffeine a habit especially in large amounts because over time it can cause adrenal burnout due to its effects on cortisol levels. That's why you're able to concentrate after you drink coffee. Ideally though you should be able to concentrate that much without any caffeine. You definitely should eat something, and the fact that you can't concentrate after you eat indicates to me that you have sluggish digestion which could be caused by many things. I'd also suggest that you go for a run or do some kind of exercise since that will increase your concentration and increase blood flow to your brain. Those are some short term things to do but if you always have trouble with digesting your food you have a bigger health problem and should read some of the posts here about how to improve your digestion.

The biggest thing that has an effect for me is caffeine. That said, I supplement with FocusFactor (there were some other brands like FocusSmart.)

Out of that the thing that seems the most effective is DMAE.

Other stuff like dark chocolate, turmeric, blueberries, fish oil, choline (or lecithin, from eggs, not soy) are great.

If you need a steady stream of ketones for your brain, you can use coconut oil - in coffee. This leaves a bit of a hot oil slick at the top, so I mix it with cocoa powder first (and that has the effect of not putting me to sleep as caffeine removes some magnesium.)

L-Tyrosine with B6 and DMAE can help. L-Glutamine in small amounts can help with sugar cravings, but you likely won't need this.

Have you tried green tea? It has caffeine, but also high levels of theanine. Supposedly the theanine leaves the cognitive enhancement effects of caffeine, but blocks the cortisol/adrenaline effects. My own experience: it is quite calming, though not as delicious as coffee.

Actually I should also say that black tea has theanine in it, although less than green.

Many people report greater concentration during ketosis or intermittant fasting. Go ahead and fast if it helps you concentrate and eat later in the day

To be honest, now is not the time to be researching this stuff. I'd say continue with what you're doing, eat at the regular times, and research/plan this out for next time. Pretty much nothing you're going to do last minute the day before will give you that much of an edge. Write away, then edit away.

Research nootropics on here, and buy them in advance for next time.

Have you tried actually focusing on your work &/or working on things well in advance of the deadline?

I realize that's not always doable, so try this: -block out all distractions. the only thing active on your computer should be your essay. -put your headphones on and put on some dubstep or similarly up-tempo repetitive stuff. -get a giant glass of water (seriously, like a pint jar). -set a timer for 1-2 hours. -go to town on your essay. thesis, outline, segues, body, conclusion.

After two hours, step away from it. leave your room/house/dorm, get some food, some fresh air, attend to basic personal hygiene. Grab a coffee and another Huge Glass of Water then revise. Rinse & repeat til your work is done.