I have dyslexia and in high school learning my lines for plays was really hard but I loved doing plays, so I recorded myself saying my lines on tape (yah, I'm old) and used double cassette to fill 2 tapes with them, then run them over night while I was sleeping. I've never used this in my adult life but it worked pretty well for my lines and I suppose maybe you could use it to learn a language?

Edit: Claude tells me I was a head of my time, apparently it works but not net new, you have to also be working on it awake, it's called 'targeted memory reactivation (TMR)": https://pmc.ncbi.nlm.nih.gov/articles/PMC12592824/

While I think it's a compelling idea that playing speech in your target language while you sleep can help, I don't think it's ever been demonstrated to work.

Having said that, that sleep is incredibly important for learning anything! I practice my language learning during the day, a little bit every day, and I prioritize getting good sleep. This is mostly just trying to go to bed at the same time every night, avoiding alcohol, and giving myself an hour before bed with low lights to read and calm my mind. When you sleep, memories are consolidated, organized, and tagged for long-term storage. I will sometimes wake up in the middle of the night and bouncing around in my mind are echos of phrases and words from my target language. I figure it's working.

Dexter, from Dexter's Lab, learned French.