Verse 5

5.  The third quarter is Prajna,  where one asleep neither desires anything nor beholds any dream: that is deep sleep.  In this field of dreamless sleep,  one becomes undivided,  an undifferentiated mass of consciousness,  consisting of bliss and sustained by bliss.  His mouth is consciousness.

"The third quarter is Prajna where one asleep neither desires anything nor beholds any dream: that is deep sleep"

Prajna has the meaning of pure consciousness and knowledge.
Prajna describes the state of deep sleep,  meaning dreamless sleep.  Without the experience of dreams or sensory input there is nothing to differentiate within consciousness.  there is no consciousness of duality,  desire or fear.

"In this field of dreamless sleep,  one becomes undivided,  an undifferentiated mass of consciousness,  consisting of bliss and sustained by bliss".

Prajna being consciousness alone and at rest the state is harmony and bliss.
The term  "sustained by bliss"  distinguishes this state of bliss from the true state of Absolute Bliss,  which is properly described as Truth,  Consciousness  and Bliss.
In this state of deep sleep,  "Prajna"  the bliss is due to the temporary forgetting of desire and so forth.  this is the bliss of ignorance,  so as said this bliss of ignorance sustains itself,  it relies on its own ignorance.
The true Bliss of Non-Dual Absolute Consciousness is the Bliss of Absolute Truth.  Within this Absolute Truth duality does not exist as the cause of the delusions of desire and fear.
Bliss is the natural eternal state of The Absolute.

"His mouth is consciousness"

This  "Self"  of Creation came into being to satisfy desire.  The attributes of this   "Self"  of mouths and limbs were the means of experiencing the satisfaction of that desire.
Without the desire or fear of the waking or dream state there only remains that  "undifferentiated mass of consciousness"  which is the experience  "mouth"  of this Prajna.