주무세요. (Jumuseyo) ==> Formal. 잘 자 (Jal ja) ==> Informal. Never heard of any slang for this, unless you wanna say "같이 자자 (katchi jaja)"...lol...which means "let's sleep together", but don't go around saying that...well, uneless it's your boyfriend/husband of course ;)

You know what? We ALSO say "같이 자자" as the meaning of "let's share my bed", "I want to share your bed"..So Korean babies/kids say that to their moms( like 엄마 같이 자자 나 무서워) when they feel scared of sleeping alone at night.

lol...thank you so much for answering..hehe :)