Here's an interesting article on exercise and memory. Even more interesting is that those with a BDNF (brain derived neurotrophic factor) blunting response - genetically found in 30% of those with European heritage - after exercise didn't improve their memory with exercise though others did.
The exercise consisted of walking or jogging 4 times a week each for at least 30 minutes.
Exercise improved anxiety on the day of exam and those that exercised regularly AND on the day of the exam had the best memory on the exam.
Would you consider now exercising on an exam day?