To sleep the dream of the just, is to sleep in peace, because your life is orderly, you have acted ethically, morally, just, so you get to your bed and sleep peacefully because you are a just person, you have no charges of conscience, fear of people, shame of anyone, you sleep in peace. Now it is used in another way, this expression of sleeping the sleep of the righteous, is like just being asleep if you do the things you must do, you are lethargic, you do not react, you are too confident and you run out of time without doing anything because of the overconfidence or you simply do not start doing anything. it's like another expression "you're selling water" you're in something else and not yours.