I got some mail from my boss. It contains:
I will tell the details tomorrow. I want to sleep on it.
What does this sentence mean?
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While I don't disagree with snailboat's answer, I don't know if it quite captures the entire essence of it.
One source defines the idiom as:
sleep on it: take at least a day to think about something before making a decision.
So, yes, we are talking about a chance to make a decision overnight. However, it's worth mentioning that the general gist of the phrase means that give yourself time to consider what you are pondering, and you give yourself time to clear your head as well.
People often see a conundrum in a new light after they've had a chance to mull it over, talk about their feelings with others, and even get a good night's rest. Have you ever had an experience where something seemed very stressful at the time, but it looked very minor by the time you woke up in the morning? Or when a decision was hard to make in heat of the moment, but the right decision seemed obvious before you were even out of your pajamas the next day? The problem hadn't changed, but your perspective had. Taking the time to "sleep on" something gives you a chance to think about it for awhile – and to not think about it for awhile, too. Many sages advise people to avoid making major decisions until they have had a chance to "sleep on it:"
Make it your rule that you won't make a firm and final decision about any added costs until you've had a chance to sleep on it. In the morning, look at everything with fresh eyes and a well-rested perspective. (William J Hirsch, Jr, 2009)
Moreover, it's more than just a catch-phrase; it's apparently been backed up by hard science:
The idea that we need to "sleep on it" when faced with a big decision is no joke. We intuitively know that sleep helps us think better, stronger, and prepares our minds for optimal functionality. A Dutch study in 2006 points to the benefits of taking in information and letting the "unconscious" mind during sleep churn through the options involved. Other experiments have also backed this finding. (Michael Breus, 2007)
Just adding my 2 cents point out that the phrase doesn't necessarily mean "tomorrow". The phrase is not to be taken so literally in terms of time; it just means, "I will take some time to think and I will allow space for new perceptions and feelings to arise". Sometimes you may "sleep on something" for a few days.