Hanosane Layaef Koach:
Is a blessing recited in thanks to G-d for returning a fresh and re-energized soul to one’s body, which is a great act of kindness. This blessing was instituted by the Geonim as a result of the physical weakness which had descended to the world.
 46/6; Rama 46/6
Other Opinions brought in Admur: There are opinions [Michaber 46/6] which question whether one may recite this blessing, as no person has the power to institute blessings after the Talmudic period, [which preceded the Geonic period by about half a century]. Practically, their argument is insufficient to negate the institution of the Geonim and the widespread custom of reciting this blessing. [ibid based on Rama ibid; and so rules Kabala] However certainly a blessing which has not been instituted by the Geonim, or was but has not become the widespread custom, may not be recited and one who recites it transgresses the saying of a blessing in vain. [Admur ibid]
Custom of the Sefaradim: The Sefaradim are accustomed, based on the Mekubalim, to recite this blessing despite the fact that according to the Michaber it is not to be said. [Birkeiy Yosef 46/11; Sdei Chemed Mareches Brachos 2]
 This blessing was instituted to strengthen the faith of the Jewish people against the heretics. [Shaar Hakolel 1/11] The Arizal explains that it is said for a person that has lost part of his levush of his soul due to sin, and in the morning Hashem returns it to him refreshed upon awakening. [Shaar Hakolel ibid]