The day and night, verily, are Prajāpati. Of them, the day is Prāṇa; the night is Rayi. Those who unite in sexual union by day, verily dissipate Prāṇa. But sexual union at night is, verily, the prescribed celibacy.

Here a correspondence is established between the day-night cycle and the cosmic principles. The day (solar, active, luminous) is Prāṇa. The night (lunar, receptive, dark) is Rayi.

The specific rule about sexual union reflects the Vedic worldview: the day is for spiritual activity, study, sacrifice. Dissipating sexual energy during the day is “wasting prāṇa.” The night, being of rayi nature, is the appropriate time for reproductive functions, and when done according to ritual prescriptions, it is considered brahmacarya (conduct appropriate for spiritual growth).

For the contemporary yogī, this is interpreted more subtly: there are propitious moments for different activities. Intensive spiritual practices are more effective at certain periods of the day (traditionally brahma-muhūrta, before dawn).

The deep teaching: every act can be transformed into yoga when performed with knowledge, discrimination and within the framework of spiritual practice. It is not the action itself, but the intention and the knowledge that accompany it, that determine its spiritual value.