Not a ton of instances but the stats lean quite heavily to the long side. Between 2003 and 2006 the upside tendency was not apparent. I was unable to look back past 2003 this morning. Below are all instances from 2006 – present.
If you find morning gap analysis like this useful, you may want to check the work of my friend, Scott Andrews over at MasterTheGap.com. I look at studies like this occassionally, but it is his primary focus.