Matsya Purana — The Attendant Hosts of the Sun and Moon: Monthly Gaṇas
सुषुम्नाप्ययमानस्य शुक्ले वर्धन्ति वै कलाः तस्माद्ध्रसन्ति वै कृष्णे शुक्ले ह्याप्याययन्ति च //
suṣumnāpyayamānasya śukle vardhanti vai kalāḥ tasmāddhrasanti vai kṛṣṇe śukle hyāpyāyayanti ca //
When the current of vitality is being absorbed into the Suṣumnā, the kalās (vital, constituent measures) indeed increase in the bright fortnight; therefore they diminish in the dark fortnight—while in the bright fortnight they are nourished and augmented.
It uses the language of waxing/waning and “absorption” (apyaya) to explain cyclical increase and decrease of vital measures (kalās); this mirrors the Purāṇic idea that growth and dissolution operate in rhythmic cycles rather than as a single event.
It implies that nourishment and strength are greater in the bright fortnight, supporting the Purāṇic practice of scheduling constructive rites—charity, vows, consecrations, and major undertakings—when growth (vardhana/āpyāyana) is naturally favored.
By stating that Śukla-pakṣa augments and Kṛṣṇa-pakṣa diminishes, it supports choosing Śukla-pakṣa for temple rites, installations, and other auspicious saṃskāras—an idea often used alongside Matsya Purana Vastu Shastra tips on selecting favorable time periods.