Sleep-learning

Sleep-learning is what millions of college students do every day involves attempts to impart information to somebody by speaking to them or playing recorded messages while they are asleep. Sleep-learning products seem to have two main purpose. The first is to condition behaviour - to prevent nail-biting, cause weight loss, enhance confidence, etc. The second is to impart information - such as foreign language vocabulary or lists of facts. Evidence as to the effectiveness of sleep-learning can be classified as "inconclusive".

Well-conducted scientific studies have produced no evidence that one can learn new facts while asleep, although some experiments have shown stimulation while asleep can aid with learning when awake. Some studies also show that classical (Pavlovian) conditioning can be performed on sleeping or comatose subjects, suggesting it might be possible to use sleep techniques to combat phobias or otherwise to influence behavior; but there are few studies in this area, and nothing to indicate you can lose weight while you sleep.

Psycho-Phone
Alois Benjamin Saliger's Psycho-Phone was one of the first attempts at marketing the idea; in 1927 Saliger claimed the effect would be the same as during hypnosis. He sold scripts for various purposes: to "inspire prosperity, inspiration, normal weight, even life extension".

Be Psychic Sleep Hypnosis
The site "sleeplearning.com" offers a bullshit audiobook "hypnosis CD" that will grant you psychic powers after having listened to it for just an hour! Whoever wrote the product summary would likely benefit from a sleep-learning CD on "How not to overpromise during the sales process":

Studies
Early experiments in learning Chinese and preventing nail-biting seemed to show some positive effect, but one weakness of these experiments was that the test subjects may have been awake and listening. The classic experiments conducted by Charles W. Simon and William H. Emmons of the Rand Corporation in the 1950s avoided this problem. They used EEGs to test if the subject was awake or asleep, and sleeping subjects were read lists of questions and answers and then tested on the results. They deduced that "learning during actual sleep did not seem possible". Following criticism that these experiments did not take into account the beneficial effect of repeating information, they conducted additional tests, with repeated lists of nouns, but still found "material presented a number of times during sleep (using an EEG criterion) cannot subsequently be recalled."

For a long time, the world regarded the Simon and Emmons studies as conclusive. Recently, a few experiments have shown that experimenters can have some influence on sleepers, though they do not match the earlier, discredited experiments. In 2012, research by Anat Arzi at the Weizmann Institute of Science in Israel indicated that classical conditioning could be used during sleep to get people to associate sounds with smells. This corresponded with earlier research showing that patients who were in a vegetative state or minimally conscious could be conditioned to respond to puffs of air blown into their eyes. This research shows that it may be possible to alter behavior in sleep to treat phobias.

Some experiments have shown that spraying smells or playing sounds or music to sleeping people can enhance the retention of facts they learnt during the day. Susan Diekelmann started this research in Germany in 2010, with subjects remembering the layout of objects in a grid; and a 2012 American study demonstrated that students performed better in a musical memory game if they had previously been played the music in their sleep. A 2015 study at the University of Zurich found that playing words to sleeping people can aid with vocabulary learning if they had already been exposed to the same vocabulary. This may be connected to results of Jan Born of Lübeck University: Born found in 2010 that a continuous 0.75 hertz current fed via electrodes into sleeping brains improved recall of the day's memories by 8%. Evidence suggests that some forms of stimulation can improve memory, but beyond that little is known.