Kale, chard, spinach, broccoli: the list of healthy dark leafy greens is a long one, and they're a vital source of vitamins in the run-up to exams. All dark greens are packed with vitamin K, which helps build pathways in the brain, as well as naturally occurring nitrates and antioxidants. You'll also get a healthy amount of B6 and B12 from some of these vegetables, which are associated with improvements in alertness and memory.