As any student knows, caffeine increases alertness and reduces fatigue—useful effects, especially when you're working late. The scientific consensus is that moderate caffeine intake yields no negative long-term effects, but sensitive individuals may experience more jitters from coffee than they want. If that's the case, try black or green tea; if even those contain too much caffeine for you, try energizing herbal teas, such as peppermint, thyme, or ginseng. The newest buzz is coffees (including decaffeinated ones) blended with herbal extracts shown to support brain function, such as ginseng, ginkgo, and vinpocetine. If you need a little extra oomph to make it through a study session, a caffeinated beverage and a high-protein snack can boost performance.
Don't, however, drink a lot of coffee the night before a big exam; the caffeine could prevent you from getting a good night's sleep. Instead, indulge in a few serotonin-producing carbs to help you snooze soundly. (And don't forget to eat protein in the morning to fire up your brain for the exam.)