Patañjali's yogasūtra 1:10 Nidrā

{1.10}

अभावप्रत्ययालम्बना वृत्तिर्निद्रा 

abhāva pratyayālambanā vṛittir nidrā

अभाव प्रत्यय आलाम्बना वृत्तिः निद्रा 

The modification[of the mind] based on the absence of [any] content [of the mind] is sleep 

अभाव - absence, nullity; masc. noun.

प्रत्यय - idea, concept, content of mind; masc. noun

आलाम्बना - support, basis

वृत्तिः - modification (of the mind); masc. noun

निद्रा - sleep; fem noun

During the time that a person is in this state - of sleep, the mind is, as it were, a void. It is blank. Evidently it is dreamless sleep that is being referred to here.  Commentators stress the fact that this is not similar to the state of samādhi, where also there is no content in the mind. 

Taimini explains this sūtrā simply. He says in samādhi, all content is deliberately removed while in a waking state - where as in sleep, the brain and the mind are disconnected, so that content is not recorded or registered. It's like a car that has been put into neutral. The engine is running, but because the gears are disengaged, it will not move. As soon as we wake up, the connection is re-established (the gears are engaged) and the brain becomes the seat of mental activity once again.