Do you know why we have two nostrils? It is not because we have two eyes, two ears, two hands and two legs, and therefore two nostrils. The reason is far different. The nostrils help to maintain body temperature. The right nostril represents the surya nadi and the left one, the chandra nadi. Yogis believe that regular practice of alternate breathing will clean and rejuvenate the vital channels of energy, thus the name nadi sodhana (purification of nadis or channels).
Medical science has recently discovered the nasal cycle, something that was known by the yogis thousands of years ago. Scientists have recently found that we don't breathe equally with both nostrils; that one nostril is much easier to breathe through than the other at any particular time and that this alternates about every three hours.
The yogis claim that the natural period is every two hours, but we must remember these studies were done on people who do not have an optimum health level.Scientists also discovered that the nasal cycle corresponds with brain function. The electrical activity of the brain was found to be greater on the side opposite the less congested nostril. The right side of the brain controls creative activity, while the left side controls logical verbal activity. The research showed that when the left nostril was less obstructed, the right side of the brain was predominant. Test subjects were indeed found to do better on creative tests. Similarly when the right nostril was less obstructed the left side of the brain was predominant. Test subjects did better on verbal skills.
Medical science has not quite caught up with the ancient yogis yet. The yogis went one step further. They observed that a lot of disease was due to the nasal cycle being disturbed; that is, if a person breathed for too long through one nostril. To prevent and correct this condition, they developed the alternate nostril breathing technique. This clears any blockage to air flow in the nostrils and reestablishes the natural nasal cycle.
For example, the yogis have known for a long time that prolonged breathing through the left nostril only (over a period of years) will produce asthma. They also know that this so-called incurable disease can be easily eliminated by teaching the patient to breathe through the right nostril until the asthma is cured, and then to prevent it recurring by doing the alternate nostril breathing technique. The yogis also believe that diabetes is caused to a large extent by breathing mainly through the right nostril.
Having said that, it is important that one learns pranayam techniques only from a good teacher. It has to be taught under supervision initially because it is a powerful technique that generates a lot of energy in the body. The practitioner should be in a position to manage that energy.