Over the last few decades longer hours have become associated with higher earnings and higher educational backgrounds http://www.nber.org/digest/jul06/w11895.ht

Over the last few decades longer hours have become associated with higher earnings and higher educational backgrounds http://www.nber.org/digest/jul06/w11895.html http://www.nber.org/digest/jul06/w11895.html (old data but a good start)