"The principle of sleep-teaching, or hypnopaedia, had been discovered"

Hypnopaedia is also known as sleep-learning, or sleep-teaching. It is the technique used to convey information to a person while he or she is asleep, normally via a played sound recording. In the mid-20th century researchers found positive results from hypnopaedia, thus sleep-learning entered the popular consciousness. However, other research suggested that sleep-teaching does not work for everyone.

In 1942, Lawrence Leshan tested the theory of sleep-learning on a group of nail-biting boys attending summer camp. The experimental group consisted of 20 nail-biting subjects, aged 8 to 14 years. For six nights, the recorded message, "My finger-nails taste terribly bitter," was repeated fifty times. After the experiment, 40% of the boys quit biting their nails. The results supported Leshan's theory that recorded messages played during sleep incite mental activity, but, because the experiment was only 40% effective, hypnopaedia was not regarded as an effective method of learning. In 1956, the studies of Charles Simon and William Emmons led to their statement that learning by hypnopaedia is “impractical and probably impossible.” Today, the theory of sleep-teaching has been largely discredited and is not taken seriously. However, many people use sleep-therapy to attempt to break habits such as smoking or over-eating. The success of these treatments is debatable.

The Theory and Practice of Sleep-teaching, by Dave Curtis.