In the article about over sleeping (http://www.straightdope.com/classics/a990205.html) Cecil said that it was pretty much a myth that people got groggy from too much sleep. But I read somewhere that there is a lot of evidence that the human body was designed to sleep at night and slow down and nap around noon. (Lots of cultures seem to live by that idea) so maybe when people over sleep they just sleep right in to their next nap period which throws everything off.