Why do we have two nostrils?

Breath and our attention to it is a major aspect of Yoga. But why? Why is pranayama crucial?

Here's what scientific studies have confirmed; 1. At any given moment you are breathing through one dominant nostril; then some time later you switch to another one. This switch happens every 2-2.5 hours (although it can vary), and continues in a rhythmical fashion. 2. This rhythm changes over time for most people. 3. Both nostrils are controlled by the central nervous system.

Yogis arrived at a similar discovery a while ago, though they have used different language to describe it. They said that our energy balance is affected by the flow of energy along three major channels: ida, pingala and sushumna nadis. Ida (lunar channel) is cooling and has overall feminine quality; it terminates at the left nostril. Pingala (solar channel) is heating and has an overall masculine quality; it terminates at the right nostril.

Breathing in through a nostril will emphasize the qualities of that channel; breathing out through the nostril will sedate the corresponding channel. Sometimes there might be too much heat in the system (for example, if somebody has hot flashes or feels angry/irritable), which can be helped by breathing in through the Left nostril (to stimulate the lunar channel) and out through the right (to sedate the solar chanel). This pranayama practice is called Chandra Bhedana. When there is not enough heat in the system (for example, cold limbs, low blood pressure, depression), it can help to breath in through the Right nostril (to stimulate the solar channel) and out through the Left (to sedate the lunar chanel). This pranayama practice is called Surya Bhedana.