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.