"That is deep sleep where the sleeper does not desire any enjoyable object and does not see any dream. The third quarter is Prajna who has the state of deep sleep as his sphere, in whom everything is undifferentiated, who is a mass of pure consciousness, who, indeed, abounds in bliss and is an enjoyer of bliss, and who is the doorway to the (objective) experience (of the dream and waking states)." 
(Mandukya Upanishad, Verse 5)

English Audio CD - 60 min.