Good news for coffee drinkers! I'm relieved my 4-5 cup-a-day habit is beneficial.
But I have to say the expression "cup of joe" is one I don't think I've ever heard. This seems to be the origin of it:
The Origin of the Phrase "Cup of Joe"
The origin of the phrase "cup of joe" goes back to the early 1900s and Admiral Josephus Daniels. Admiral Daniels was secretary of the navy in the Wilson administration during World War I . After he outlawed alcohol on ships, coffee becaome the favorite beverage of naval crews. Hence the name "cup of joe."
But there are other theories: http://h2g2.com/dna/h2g2/A1300410