Sleep learning doesnât work.
To my students sleeping in the front row of my on-campus class, my voice is around 50dB. But their only awareness is its presence or absence. When the voice becomes absent at the end of class, they all of a sudden arise and exit the room.
Having done this my self, in my younger days, I know that the level of perceptual organization they achieve, and information they have obtained is not enough to pass a test...although they look rested and appear to feel great.
I actually once tried to learn Russian this way--by listening to Russian records in my sleep. The records were well above my hearing threshold but my level of attention was obviously subliminal.
After a month, it happened that a friend who spoke Russian came up and said "Good Morning, Hall," in Russian. To my credit, I recognized that it wasn't English, but my only tangible response, after month of "sleep learning," was an overwhelming feeling of drowsiness.