Happiness is trending downward. That's what University of Vermont scientists have found by analyzing the "emotional temperature" of tweets from 63 million Twitter users since January 2009. Here's what you should know:

How was the study conducted?
Researchers analyzed more than 46 billion words from the tweets of 63 million people. Every word was given an "emotional temperature" on a scale of one to nine, with nine being the happiest. A word like "laughter" would get a ranking of 8.5, while "terrorist" got a 1.3. Once all the words were ranked, researchers looked for patterns related to the time, date, or location the tweets were posted from.

What did researchers find?
Happiness, at least amongst people on Twitter, has been on the decline since April 2009. And unsurprisingly, people are happier on the weekend and less happy early in the week. Twitterers would also seem to be happier at night than they are in the morning. Researchers also noted drops in happiness related to specific events, like the tsunami in Japan.

Okay, but is Twitter really a good indicator of happiness?
Researchers think so. Twitter lets us look over the "collective shoulder of society," says the study's lead author, Peter Dodd. "Everything we say or write is a distortion of what goes on inside our head." He also notes that while Twitter once skewed young, it's increasingly ubiquitous among all age groups. But of course, happiness isn't everything. "It might well be that we need to have some persistent degree of grumpiness for cultures to flourish," Dodd says.

Sources: PC Mag, Psych Central