There are only 24 hours in a day, and usually about a third of that is spent sleeping. So is it possible to make use of this time and learn a new skill or even a language? In other words, is sleep learning possible?  

 Many studies have found that a basic form of learning, called conditioning, can happen during sleep. In a 2012 study published in the magazine Nature Neuroscience, for example, Israeli researchers found that people can learn to connect sounds with smells during sleep. The scientists played a tone ( 声音 ) to sleeping study participants ( 参与者 ) while putting some rotten ( 腐烂的 ) fish beside their beds. Once awake, when hearing the tone, the people held their breath in anticipation ( 预判 ) of a bad smell.  

"This was a clear finding showing participants formed new memories during sleep," said Andrillon, a scientist in this field.

 Although the new memory was implicit ( 暗示性的 ), it could influence how people behave, researchers found in a 2014 study published in the Journal of Neuroscience . In that research, after spending a night in a room full of cigarette ( 香烟 ) smell mixed with rotten eggs or fish, smokers use fewer cigarettes.  

 Andrillon and his team members have found that learning in sleep can also go beyond simple conditioning. In their 2017 study published in Nature Communications, participants were able to pick out sound features that they had heard during sleep.  

Learning abilities in sleep may spread to learning of words. In a study published in Current Biology, researchers played pairs of made-up words and their supposed meanings, like "guga" means elephant, to sleep participants. After this, when awake, the participants did better in picking the right translation of made-up words in a multi-choice test.

 So far, research suggests it may be possible to learn about the tone and pronunciation of a language or even the meaning of words while sleeping, although it is to a weaker level than what we do during the day without noticing.