-- We tweet what we feel.
An innovative study that used Twitter streams from 2.4 million people around the globe to take their emotional temperature found that people start the day in a good mood. But it decreases as work starts, then improves when work ends.
People were happier on the weekends, perhaps because their morning good mood started two hours later, indicating that they may have been sleeping in. In the United Arab Emirates, where the week runs Sunday to Thursday, the same pattern of workweek and weekend applied, but on different days.
The research, published Thursday in the journal Science, is an important part of an ongoing revolution in the social sciences, says Nicholas Christakis, a professor of medical sociology at Harvard Medical School. Instead of talking to tiny numbers of subjects, researchers can follow the digital expressions of millions of people. "It's a whole new way for social scientists to understand human beings," he says.
It's possible to tease out all sorts of fascinating data from Twitter, says Michael Macy, senior author on the paper and a professor of sociology and information science at Cornell University in Ithaca, N.Y.
Macy and his co-author, Scott Golder, examined 509 million messages sent between February 2008 and January 2010 in 84 countries.
Working with English-only tweets, the researchers used a list of words that measure behavioral and psychological dimensions called the Linguistic Inquiry and Word Count. Words such as "happy" and "enthusiastic" were tagged for "Positive Affect," and words like "depressed" or "anxiety" for "Negative Affect."
That people were in such a good mood first thing in the morning might seem surprising, Macy says, as many people claim to be grumpy at that time. But in fact it's likely linked to the amount of sleep they've gotten. "People wake up refreshed, and then their mood is elevated at breakfast time," he says.
Environmental effects are also evident. Work, it seems, makes people less happy. "People are happier on the weekend, and that perhaps could be attributed to the fact they're engaged in activities that are more enjoyable than work," Macy says.
People could also be happier on the weekends because they're getting more sleep. The pattern of when they begin tweeting is delayed by about an hour and a half or two hours, which suggest that people are sleeping in on the weekend. So it's possible that people's better mood on the weekend has to do with that they woke up later and they're not as tired, Macy says.
It's also possible they're in a better mood because they're more likely to have been able to wake naturally "versus waking up to an alarm clock when their body is not ready to wake up naturally," he says.
"It's an exciting paper" says David Lazer, a professor of political and computer science at Northeastern University in Boston. The number of participants is big enough that the findings should be robust, though Lazer notes that it's possible that "people who tweet a lot are distinctive in some ways," perhaps in terms of how much sleep they get. "For example, they must have enough time to tweet."
The next study would need to look at how different tweeters are from non-tweeters. "Maybe they're more homogeneous across national boundaries," Lazer suggests.
The study also provided validation for why "the winter blues" really are about winter and not about the amount of sunlight people are getting.
Research recently has begun to disprove the popular "Seasonal Affective Disorder" hypothesis, which suggested that the shorter, darker days of winter and the absence of bright sunlight contribute to depression.
Instead, some scientists now say the daily shortening of the days, rather than the absolute amount of light, is what causes the depression.
The Twitter study found confirmation of this in that people were more likely to have negative affect in the winter and less likely to in the spring — even on days when the length of day was exactly the same. But in the winter people knew the next day would be shorter and darker, whereas in the spring they knew the next day would be longer and brighter.
"We find is that during the time of the year when the days are getting longer, people's affect is more positive then during the period is when the days are getting shorter," Macy says.