Washington: They may be short and limited to just 140 characters, but the messages you post on Twitter can tell something about your state of mind, researchers say.

Using values assigned to words contained in 4.6 billion messages, also known as tweets, posted from September 9, 2008, to September 18, 2011, the researchers from the University of Vermont were able to track how levels of happiness rose and subsided among English-speakers around the world.

It turns out the last months of each year, Saturdays and the early mornings are happy times, while January, the first days of the week and late-nights are not, agency reported.

Holidays and individual events, like a royal marriage or the death of a celebrity, can also have significant effects on mood, the team found.

Previous research examining global mood swings through the lens of Twitter has found similar patterns. But recently, the overall trend is a gloomy one, the researchers said.

"After a gradual upward trend that ran from January to April, 2009, the overall time series has shown a gradual downward trend, accelerating somewhat over the first half of 2011," they wrote in a study appeared in the journal PLoS ONE.

Average happiness, they found, tends to increase during the last months of the years studied, dropping off in January. Happiness also rose over the weekend, peaking on Saturday, then reaching its nadir on Tuesday.

When all of the scores were averaged together, the word "laughter" ranked 8.5, while "terrorist" got a 1.3, for example. The team then applied these values to words contained in the tweets, which also carried date and time, and sometimes other demographic information, to get a sense of the collective mood.

This allowed the researchers to look over the "collective shoulder of society," said Peter Dodds, a mathematician at Vermont who led study.

The team also found that daily profanity use followed a pattern roughly opposite to that of the daily happiness cycle; for example, an analysis of five common expletives revealed that their use tended to peak during the late night, then plummet in the early hours of the morning, the hours when average happiness rose.

There were also days when average happiness rose or sank out of pace with nearby dates. Among outlying dates, happiness was highest on Christmas Day, followed by Christmas Eve. In general, it appeared people were much happier on the holidays.

The only singular, non-annual event to stand out as a positive day was that of Prince William and Catherine Middleton's wedding on April 29, 2011, the researchers wrote.