A few minutes or an hour ago, an inspirational muse pounded my head with her hammer of enlightement and TA-DAH! a theory formed up in my head. This is called the theory of HAIRY KNOWLEDGE.

Let's assume knowledge has mass. Therefore the more knowledge we absorb (by studying mainly), the more mass gets into our head. However, since it is biologically unnatural for our head to expand proportionallyto the amount of knowledge absorbed, the knowledge is "pushed" into manufacturing the hair instead. Hence, causing the hair to grow (since we are learning everyday) and your head to get heavier.

Digression
If you take a look at portraits of great Mathematicians, you will notice that all their hair is brushed back. Reason: To cover the lack of hair at the back due to scratching over complicated mathematical problems that will BLOW YOUR MIND.

Follow up: Read "Hair Loss Theory"