The opposite meaning of ゆっくり is いそいで. ゆっくり is "take it easy." or "not in haste". いそいで is "hurry up!" They can be used for various verbs. "sleep long" and ゆっくりねる is almost the same meaning. ゆっくりあるいてね = Don't walk in haste to avoid getting injured. いそいであるいて! = Walk hurry up, otherwise you'll miss the train. So guess what the purpose of ゆっくりねる and いそいでねる. The answer is ゆっくりねる is for taking enough of rest, and いそいでねる is because it's late at night or you need to get up early.

It means to sleep well. ^ ^

Thank you :)

ゆっくりして、休んでください。 Take it easy, rest well. ゆっくり休んで、元気になってね。Rest well and I hope you feel better. It can also be used in this context.