The Prashna Upanishad with Shankara’s Commentary
अहोरात्रो वै प्रजापतिस्तस्याहरेव प्राणो रात्रिरेव रयिः प्राणं वा एते प्रस्कन्दन्ति । ये दिवा रत्या संयुज्यन्ते ब्रह्मचर्यमेव तद्यद्रात्रौ रत्या संयुज्यन्ते ॥ १३ ॥
ahorātro vai prajāpatistasyāhareva prāṇo rātrireva rayiḥ prāṇaṃ vā ete praskandanti | ye divā ratyā saṃyujyante brahmacaryameva tadyadrātrau ratyā saṃyujyante || 13 ||
13. The day and night is, indeed, the lord of creation. Of that, the day is the prâna and the night, indeed, is the food. Those who combine with Rati (sexual intercourse) by day, spill prâna. That they combine with Rati by night is Brahmacharyam indeed.
Com.—And that lord of creation, i.e., the month, culminates in his component parts, the day and night as before explained. Of him, the day is, indeed, prâna; the eater, the fire; the night, indeed, is food, as before explained. Those spill their prâna, i.e., the day, that is, waste it by separating it from the selves. Who? Those ignorant men, who by day have carnal intercourse with woman, who is the cause of sexual pleasures. This being so, the prohibition that it should not be done is a rule laid down by the way. If they have intercourse by night, in season, that is Brahmacharyam indeed. That being praiseworthy, the mandate that one should approach his wife during rita, in season, is also, by the way, enjoined. What is relevant here is this, i.e., the lord of creation in the form of day and night becomes such as corn, grain, etc.