Never underestimate the power of a good night's rest -Rajesh Jain:

SKIMPING on sleep does awful things to your brain. Planning, problem-solving, learning, concentration,working memory and alertness all take a hit. IQ scores tumble. If you have been awake for 21 hours straight, your abilities are equivalent to someone who is legally drunk.

Luckily, it's reversible - and more. If you let someone who isn't sleep-deprived have an extra hour or two of shut-eye, they perform much better than normal on tasks requiring sustained attention, such taking an exam. And being able to concentrate harder has knock-on benefits for overall mental performance. Attention is the base of a mental pyramid, if you boost that, you can't help boosting everything above it.