In his own study, Macy analyzed words expressing mood in 500 million tweets sent between February 2008 and January 2010. He found that people’s mood, as measured by their tweets, tended to be elevated in the morning and to decline as the day progressed. Weekends tended to be “happier” days, although mood peak started later in the morning, possibly reflecting twitterers' tendency to sleep in. Most remarkable, Macy said in an interview with Science magazine, was that the tweets showed a similar pattern across the 84 countries where the tweets originated, suggesting that the daily mood curve is fundamentally human rather than cultural.
Nervermind the fact that looking only at the universe of twitter users inherently skews the results...