अभाव – प्रत्ययालम्बना वृत्तिर् + निद्रा  or  अभाव – प्रत्ययालम्बना तमोवृत्तिर्निद्रा

abhāva-pratyayālambanā vṛttir + nidra or abhāva-pratyayālambanā tamovṛttirnidra

When the mind is in a state of deep sleep one could consider all patterns, fluctuations (vṛttis)  to have ceased, but is this not cittavṛtti-nirodhaḥ (छितवृत्ति निरोध​ः) as described in sutras I:2, 3, & 4.  Although the mind may seem to not be whirling or taking the form of any pattern, in the state of nidra the guna of tamas (heaviness, solidity, denseness, lethergy) is dominant. In the cittavṛtti-nirodhaḥ which leads to the seer seeing itself, and all vṛttis ceasing, the guna of sattva presides. Please note that there are two versions of this sutra, one where the guna of tamas is implied and one where its presence is stated. 

(For a little more information on the gunas look at the Yoga, Sankyha, Vedānta pdf in the “What is Yoga?” section of the website)

A literal translation of this sutra could be:

absence of impressions supporting the mind stuff is sleep

or

absence of impressions in the mind stuff caused by tamas is sleep.

ālambanā is a support, it can refer to the object which has been chosen as the focus of concentration/meditation. For example if I focus my awareness on the North Star then the North Star is the support, the ālambanā.

Pratyaya refers to a cause or something that forms the basis for something else, it can also mean an imprint that is made by an external object on to the mindstuff, like the negative of a photograph which needs to be developed, in this case the pratyaya is ‘developed’ by the buddhi and then presented to purusa. Both of these terms will reoccur throughout the sutras.

Patanjali differentiates between nidra, deep sleep, and swapa, the dream state. The dream state is related to the next pattern for the mind to follow, smṛtiḥ, memory.