I read about this in the applied linguistics courses I took in uni (my minor). Basically, it's useless, except that it might give you some sense of the language's sound system. But it will seriously impede your language-learning ability in the sense that it gives you a poorer sleep, which affects long-term memory retention.

Exhaustive research (mostly done in Holland, Scandinavia, and to a lesser extent India, where polygots are common) has basically proven what modern people seem to find impossible to accept, but people even sixty years ago considered a basic fact of life: best way to learn a language is to pull your socks up and put some effort into it.

HA! I lookd into this a few months ago (I think I posted a similar question) and found research that shows it does not work. Something about the parts of your brain used for learning are, well, sleeping when you sleep.

I did try it though. More for recognition of pronunciation, or to get used to/familiar with the syllable sounds and tones and stuff. I don't think it worked either. LOL  It might, however, have been the cause of several weird dreams ;) 

Let us know if you try it and how it works for you. 

You are better off doing this while you are awake.  Brains have more important things to do when we are sleeping.

Learning a language while sleeping will not work for several reasons. There is a time to do something and when you are sleeping that is definitely not the time to do it. Would you really want to hear some girl or guy talking in your ear about something? I wouldn't! 

Take the advice of what they use to teach you at school. Go to bed early eat a healthy breakfast so you will do good at school. If you turn on some device with all the static and whatever else it may cause it will just cause you to have sleep deprivation and you may catch some kind of insomnia. When that happens it will be harder for you to learn anything let alone a new language that is as hard as learning Mandarin Chinese.

Yeah its not going to do anything except give you some freaky dreams. You turn on the lesson for learning directions, and then find yourself being chased by a giant monster who keeps asking "where is the bathroom?" "how do I get to the subway?"

I think the "learn while you sleep" approach can actually work for some people... My father learned German this way. (Yes, in addition to studying during waking hours.) I attempted this with the Mandarin lessons I have (the name of the company escapes me at the moment).

I have insomnia, so I'm virtually awake all night anyway, but I found that the pauses between the speaking (so that the listener can repeat the words being learned) actually disturbed me worse than the talking.

I don't really have the time/energy to go through and edit the spaces out of the lesson files, so I've given up this experiment for the time being, but I think listening to recordings while one sleeps can be useful, in addition to day-time studying.

besides... weird dreams are fun.