Gleanings from the Talmud - Berachot 6b
Four our second entry for the same daf, there is a comment mentioned in Berachot 6b concerning what happens to souls during sleep. There is a common saying in Judaism that sleep is 1/60th part of death meaning that sleep is like death, only not nearly as severe, of course. It is possible that this is the origin of that belief. This is another point that I will investigate further.
From footnote 49 on daf Berachot 6b5: When the souls of people rise to Heaven at night, God inquires: "Who is there among you that fears Hashem, etc.?" (Ben Yehodaya).
This implies a Judaic belief in that sleep is the souls ascension to heaven and that it is restored to the body upon waking. This most likely is the source of the morning blessing for having returned an individual's soul.