The Law Code of Yājñavalkya 3.3.40

3.3.40

āhāravihārāvasaravarjyaṃ rātrau śuciḥ prayataḥ svapyāt nopaviśen nāpi tiṣṭhet | divāsvapnasya puruṣamātrārthatayā pratiṣiddhatvān na tannivṛttiparam | tathā bhūmāv eva svapyāt | tac ca bhūmāv eva na śayyāntaritāyāṃ mañcakādau vā | dinaṃ tu saṃprapadair aṭanair nayet | sthānāsanarūpair vā vihāraiḥ saṃcāraiḥ kaṃcit kālaṃ sthānaṃ kaṃcic copaveśanam ity evaṃ vā dinaṃ nayet | yogābhyāsena vā | tathā ca manuḥ |

Except during the time of eating and roaming about, he should sleep at night, pure and restrained; he should neither sit nor stand. Since sleeping during the day is prohibited for all persons as such, this [instruction] does not aim at precluding that. Likewise, he should sleep on the ground alone; and that means on the bare ground, not on a bedstead or a cot placed between. But he should spend the day moving about on the tips of his feet; or he should spend the day through activities consisting of standing and sitting, that is, standing for some time and sitting for some time; or through the practice of yoga. And so Manu says: