A word of warning on the power/protein bars....the energy bars with high protein can trigger serious stomach distress! Gas, an urgent need to go to the bathroom, or both. It seems to build up over time with some bars, but many can cause rather embarassing (and odiferous!) gas right away. I had a co-worker who used to eat them for breakfast, and whew!! LOL.
So try those out for at least a week to see how they affect you. But most definitely do NOT eat more than one a day.
I didn't do the PB&J trick, but a breakfast or lunch (whichever meal you're eating right before your test) that's focused more on protein than carbs will give you great mental stamina. Tuna works the best for me for lunch, and eggs for breakfast. I wouldn't recommend bringing either of those to the test center, though, so a PB&J is a good idea!
I am hypoglycemic and have to keep my blood sugar levels even. My best snacks: an apple, nuts (almonds, peanuts or cashews), granola bar, yogurt, and cottage cheese.
Make sure that whatever you bring as a snack, that you've eaten it enough before that you don't find out the hard way that it gives you a headache (nuts can do that) or upset your stomach. My first test was a disaster because I loaded up on too much protein to try to sustain my energy (I'd been sick as a dog for a week with an ear infection and was loopy), and had an upset stomach throughout the test. Needless to say, I bombed!!
