That's what life's like according to Twitter.

A new study in the journal Science examined the contents of more than 500 million Twitter messages sent in 84 countries over the course of two years, looking for signs of good moods and bad. It found what a lot of us could tell by looking at our own lives.

Optimism is reborn with each new day and slowly erodes as we work, study and go about our quotidian affairs. Our mood lifts as we head home to friends, family, entertainment and beer. Our outlook tends to be sunnier on weekends. And speaking of sun, when it starts to pile up in the spring or disappear in the fall, that affects our mood, too.

The fact that two researchers at Cornell University confirmed such obvious truths across cultures using Twitter as their data source is the other -- and possibly more important -- finding of the study.

"This is a stone in the foundation of a new social science that is being built," said Nicholas A. Christakis, a sociologist at Harvard University who was not involved in the research. " We're in a similar place that we were in in the 17th century with the discovery of the telescope and microscope."

The study isn't the first to use "data-mining" of social media for scholarly purposes. A study presented at a conference in Hyderabad, India, last spring analyzed how information flows through Twitter networks of celebrities, bloggers, organizations and media outlets. The Cornell study goes beyond that to examine the emotional state of millions of users.

The research community has not yet judged whether such a sample -- non-random, English-speaking, heavily tipped toward the young, well-educated and talkative -- is a reasonable surrogate for humanity as a whole. But the fact that it gives predictable answers suggests that it may be.

Other experts wonder if knowing a person's or a population's emotional state tells you much. "The real problem with this method is that you don't know what the people are doing," said Jonathan Gershuny, a sociologist who directs the Center for Time Use Research at the University of Oxford. " All you know is they're on their social network sites. The real job is to find out what has got them steamed up."