The Royal Women's Hospital has some good advice about what to eat when you're studying for exams. They suggest:
1. Eat small, frequent meals.
2. Easy and nutritious foods such as frozen dinners, tinned soup, peanut butter, cheese, tuna or chicken sandwiches and eggs on toast.
3. Snack on yoghurt, nuts, dried fruit, fresh fruit, plain popcorn, veggie sticks with dip.
4. Limit coffee or tea intake to two cups per day.
5. Drink plenty of water.
You can read more here.