dk82 - 8:54 pm on Dec 6, 2012 (gmt 0)
The trend you're experiencing is most likely related to time zones. Google's reporting is based on U.S. Pacific Time. Depending on your niche, most of the traffic during early morning hours (i.e. 12-8a) is likely outside of North America and may pay less than U.S. advertisers. You could isolate this by using the Countries reporting in AdSense.