Sleep-learning, also known as hypnopedia, is the concept of acquiring knowledge while asleep. It may sound like a fascinating idea, but it's important to note that the effectiveness of sleep-learning is highly debated among experts. Nevertheless, there are various synonyms for sleep-learning that can help broaden our understanding of the concept. Some of these include subconscious learning, passive learning, and osmosis learning. These synonyms highlight the idea that sleep-learning involves absorbing information without active participation or conscious processing. It's important to note that while sleep-learning may have limited effectiveness, it's still an intriguing concept that continues to be explored by researchers.