"In connection with the science of chakras (Laya Yoga), we have said a little about the three principal nadis - pingala, ida, and sushumna - and have said that ida and pingala originate at the base of the spine and terminate in their respective left and right nostrils, but sushumna covers the whole area, from the base of the spine to the top of the corpus callosum. In Swara (Breath) Yoga we get a very clear understanding of these three principal nadis. Ida is most active in operation of the left nostril, pingala with the operation of the right nostril, and sushumna when both nostrils operate simultaneously.
Sushumna automatically operates at dawn or dusk, when the nostril connected with the planets stops and the nostril of the day takes over(...The nostrils obey the sun in that they change about a half an hour before sunrise. The same nostril that starts the day also ends the day, at sunset. The nostrils also corespond to the planets. The right nostril, which is masucline (solar) and connected with the left hemisphere (of the brain) is connected with the solar planets: the sun, Mars and Saturn...The left nostril, which is
feminine (lunar) and connected with the right hemisphere, is associated with the lunar planets: the moon, Mercury, Jupiter, Venus...)...Even if the nostil of the planet is the same as the nostril of the day, sushumna starts working at the time of sunrise.
...Sushumna is meant only for calming the system... But it is the best nadi for Yoga and meditation. In Hatha Yoga, alternate nostril breathing is practiced to open sushumna and is prescribed for five minutes before starting meditation...At sunrise and sunset it operates for a longer duration, which can be prolonged by meditating at those times of day. This was one of the reasons why the seers of the Vedas, Upanishads, and Tantra prescribed worship for the times of dawn and dusk..."
"Tools for Tantra" Harish Johari