우주잠 is a term to describe a type of sleep (a deep sleep). When you sleep so well (deep enough) that you feel fully recovered afterwards, you can say 방금 우주잠 자고 나니까 피로가 다 녹아내렸어요 (I just had a 우주잠 and I feel fully recovered from the tiredness that I had before).
Remember, however, this is a type of slang term. Even most Koreans wouldn't know what this means (especially older people). So if a foreigner who is not fluent in Korean uses this term to Koreans, they'll probably think you are trying to say 우주 정거장 (space station).