Final answer:
Early sleep-teaching experiments were abandoned due to a lack of efficacy and understanding of the sleep process, as well as ethical concerns.
Step-by-step explanation:
Early sleep-teaching experiments were abandoned primarily due to their inefficacy, ethical considerations, and the greater understanding of sleep and its various stages gained from research. These methods, known as hypnopedia, were premised on the belief that learning could occur during sleep; however, subsequent studies have shown that the sleeping brain is not in an optimal state for learning new information, especially as different sleep stages serve distinct functions.
Furthermore, there was a lack of evidence to support the long-term retention of information presented during sleep. Ethical issues also arose, considering the non-consensual aspect of imparting information to individuals while they were not fully conscious. Plus, the realization set in that sleep's primary functions—physical restoration and memory consolidation—could be compromised by such interventions.