The professors analysed words from 2.4 million internet blogs, including Twitter. They awarded a score out of nine depending on how positive the words were.
Negative words such as betray, cruel, hatred and suffocate all scored less than two, while positive words such as free, fun, and rainbow scored above eight.
As might be expected Saturday and Sunday rated predictably well. Surprisingly Monday was the second happiest day of the week - although given the testing methodology it is likely that people were writing about the preceding weekend.
Professor Danforth said: "People's daily experience is being reflected somewhat in what they're saying. And this is more of a story, we think, for Twitter messages, which we've just started to pay attention to, about a year ago.
"They think they are communicating with friends, but since blogs are public we're just looking over their shoulders.
"The weekends tend to be fairly happy and Wednesday turned out to be the saddest day.
"What we're attempting to do is measure collective happiness on a much larger scale, similar to measuring the temperature outside. The energy of a few molecules bouncing around doesn't give a good indication of heat, you need billions or more."
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