I think it's "money never sleeps", and that's because there's always a market open somewhere since we're not all in the same time zone. Your local stock market might be closed, but people do trade globally. Speaking of this particular phrase, if you haven't seen Wall Street 2: Money Never Sleeps, check it out. It's a pretty good picture.
If my memory does not fail me the British Empire was called the Empire in which the sun never sets a reference meaning that the empire was so big and extensive that some part of its territory was always illuminated by sunlight, the markets are the same, an investor in the US is not limited to only invest in the US markets he can invest in different markets across the globe so there are always opportunities to make money no matter what time it is.