"The two nostrils are associated with two very different energies. When we breathe through the right nostril, we are energized and stimulated. When we breathe through the left nostril, we relax and calm down. Our breath naturally changes dominant nostrils approximately every 2-1/2 hours. After eating our nostrils will change to the left to accommodate the energy needed to digest our food. That is one reason why we feel like sleeping after eating.
You can tell which nostril is your dominate one at any time simply by blocking off one, then the other. The dominant one is easy to breathe through and the less dominate one feels like it is blocked."
-- I, for one, found this fascinating :)