Manava Dharmashastra 1.65

1.65

ahaśca rātriśca te ahorātre | tayorvibhāgaṃ karoti ādityaḥ | udita āditye yāvattadīyā raśmayo dṛśyante tāvadahanryavahāraḥ | astamite tu prāgudayādrātrivyavahāraḥ | manuṣyaloke devaloke vā | yatra tarhyādityo na vyāpnoti raśmibhistatra kathamayamahorātravibhāgo vijñeyo'ta āha | rātriḥ svapnāyeti | svayaṃ prabheṣu bhūteṣu nityaprakāśitvātkarmaceṣṭākāryārambheṇa svāpena ca vibhāgaḥ | yathaiṣadhīnāṃ niyataḥ prādurbhāvakālaḥ svābhāvyādevaṃ karmaceṣṭāsvāpāvapi kālasvabhāvata eva niyatau |

The Sun divides the ‘Day’ and ‘Night’ of Men and Gods; [of others] what is conducive to the repose of beings is ‘Night,’ and what is conducive to activity is ‘Day.’—(65)

Medhātithi’s commentary (manubhāṣya):
‘Ahorātra’— ‘day’ and ‘night’; of these the Sun makes the division; the Sun having risen, so long as his rays are visible, it is called ‘day’; and from the time that the Sun sets to the time that he rises again, it is called ‘night.’ Such is the case in the region of men and in the region of Gods.

“In a place where the Sun does not reach with his rays,—how is one to make this division of ‘day’ and ‘night’?”

The answer to this is given in the second line—‘what is conducive &c.’ For such beings as are self-illumined, as the light is always there (and it never ceases) the division of ‘day’ and ‘night’ is made according to the undertaking of actions and going to sleep. Just as in the case of herbs, the time for sprouting is fixed by nature, exactly in the same manner, in the case of the beings in question, the times of ‘activity’ and ‘repose’ are fixed by the nature, of time itself [and are not variable].—(65)