Right from Lord Brahma (the creator) to all the creatures, all are Anandis, i.e. those who experience Ananda (happiness). This experience is got only through an instrument like mind, brain etc. But there are no instruments in deep sleep. Therefore, the supreme happiness (Paramananda) there is natural. There is no experience of Ananda there. There is no division of Ananda and Anandi (one experiencing Ananda).
The sleeper is Ananda itself, not one experiencing it. Actually, this Paramananda is always there in the cave of our heart (hrdya guha), but it is generally covered by darkness. When a desired object comes into contact, the ensuing mental form clears this mental covering and the already existing Ananda is just felt. But this mental form is not stable. Therefore, such a happiness is only momentary.
For permanent happiness we need to be in constant realisation of our inherent nature as Ananda. This is achieved through Vedanta Sadhana under the guidance of an able Guru. The purpose of this analysis done by the Upanishads is to demonstrate that permanent happiness is not an abstract promise, but a fact which can be glimpsed everyday in our deep sleep.