Patañjali's Yogasūtra I.10

Yogasūtra of Patañjali | Viniyoga with Dr Kausthub Desikachar

Dr. Kausthub Desikachar is the current lineage holder of the Viniyoga® tradition worldwide. He succeeds his father TKV Desikachar, and his grandfather T Krishnamacharya. His objectives include the sharing of the authentic teachings of Yoga to the modern era, as well as building bridges between different healing modalities to promote physical, emotional social and spiritual health.

I.10 - abhāva-pratyaya-ālambanā tamovṛttiḥ-nidrā

Deep sleep is the absence of all other activities of the mind and is dominated by Tamas.

Sleep is a common activity for the mind and a regular state for all living beings. There is a certain time for it when it is appropriate. The heaviness that induces sleep can also arise from boredom or exhaustion.