According to the article: Drawing on messages posted by more than two million people in 84 countries, researchers discovered that the emotional tone of people’s messages followed a similar pattern not only through the day but also through the week and the changing seasons. The new analysis suggests that our moods are driven in part by a shared underlying biological rhythm that transcends culture and environment.

Read about the study here: Moods on Twitter Follow Biological Rhythms, Study Finds – NYTimes.com.