It is often said that if we dream in a foreign language, it's a sign that we are making progress in learning that language. But is it true? 

Before we can look at multilingual （多语言的）dreams, first we need to look at sleep. The link between sleep and language can be applied to how we learn any language, including our mother tongue. Even adults still learn about one new word every two days in their first language, but, if we are going to have a firm grasp of that new word, we need to link it to our existing knowledge. "In order to do that, we need to have some sleep", says Gareth Gaskell, a psychology professor at the University of York.

It's during sleep that the integration of old and new knowledge happens. At night, one part of our brain—the hippocampus-takes whatever new information it got in the day, and passes it on to other parts of the brain to be stored. The role that dreams play in this night-time learning process is still being studied, but it's entirely possible that during multilingual dreams, the brain is trying to connect two languages, according to Marc Zuist, researcher at the University Hospital of Psychiatry in Switzerland. 

So having multilingual dreams could mean that our brain is trying to master a new word or phrase, but it could also have an emotional significance. . Danuta Barker, a professor of psycholinguistics（心理语言学）at the University of Silesia in Poland, suggests that multilingual dreams can express fears and desires around learning a foreign language, including the wish to be a native-like speaker or to be accepted within a certain community. 

We clearly still have a lot to learn about multilingual dreams, but one thing seems certain: if you're trying to learn a new language, you should definitely sleep on it.