The ancient books have called Sirsasana the king of all asanas and the reasons are not hard to find. When we are born, normally the head comes out first and then the limbs.  The skull encases the brain, which controls the nervous system and organs of sense.  The brain is the seat of intelligence, knowledge and discrimination, wisdom and power.  It is the seat of Brahman, the soul. A country cannot prosper without a proper king or constitutional head guide it;and so also the human body cannot prosper without a healthy brain.

 BKS IYENGAR  Light on Yoga