"Retention (of breath) should be practiced perfectly four times a day:
early morning, midday, evening and midnight, so that retention is
gradually held up to eighty (counts in one sitting)."
Hatha Yoga Pradipika (Chapter 2 verse 11)
"...For the average person it is sufficient to practice once a day. It is best to get up early in the morning, take a bath, do neti/kunjal, then asanas and pranayama. If you also have the opportunity to do sadhana in the evening, then do it befor eating.
The specified times for practice are important in correlation to body rhythms and solar/
lunar activities. At these times there is a changeover of body and external energy rhythms, and sushumna nadi is more likely to become active.
Early morning means an hour and a half before sunrise and it is called 'brahmamuhurta', 'the period of Brahma'. At this time the subconscious mind is active and if you are sleep
ing then, you will most likely be dreaming. It is a time when unconscious experiences are more likely to manifest.
Evening, around the time of sunset, is called 'sandhya'. Sandhya is tne meeting of day and night. It represents the time when ida and pingala merge with sushumna in ajna chakra. The external influences of the sun setting or the evening commencing affect body rhythms and functions, making it conducive to the merging of the pranas in sushumna.
At these specified times there is a change in the levels of energy in ida/pingala. During each cycle of ida/pingala there are high and low phases when energy either reaches a peak or an ebb. In between each cycle there is a period when the energy is stable and,
therefore, it is the most appropriate time for sadhana and breath retention. So times re
commended for the practice of retention have been chosen on account of the biological and pranic activities that are taking place then."
(Commentary on Hatha Yoga Pradipika by Swami Muktibodhananda)
"The best time to practice pranayana is at dawn when the body is fresh and the mind
has very few impressions."
"Asana Pranayama Mudra Bandha" Swami Satyananda Saraswati
"In the summer, do not practice Pranayana in the afternoon or evening. In the cool
morning hours you can have your practice."
"The Science of Pranayama" Swami Sivananda