You know, I can't find the original article, but there are a ton of articles if you search for it. Here's a good one: http://www.adweek.com/news/advertising-branding/whats-best-t...

The point that the article I read made is that it's not so much about when an email gets Opened, but when the response rate is the highest, and that occurs mainly during low traffic times, such as late at night, and on weekends. It's more about being one of a few "unread" than being available during peak traffic times.