I'm not 100% sure what they meant by that in your case. A common one that may have a similar meaning would be when someone says "sleep on it" meaning don't make a decision right now, once you let some time pass you might change your mind. I'm guessing they just mean something like "it's good luck to wait before doing your activity". Hope this helps

"sleep on something" is a slang meaning making a decision later, sort of like "play it by ear", because there is a chance that when time changes, your decision changes. But I still don't get it , why is "sleep on" good luck?

Question correction: "What does it mean exactly?" (No 's' for 'mean', because there's already one for 'do'.)