If social networking site Twitter is a barometer for the world's mood, we are content in the morning but progressively grumpier once at work.
Researchers at Cornell University in New York monitored 2.4 million people's attitudes in 84 countries over two years, and found that people of all cultures displayed similar mood rhythms.
In the study published in the journal Science today, analysis of 509 million tweets discovered there were two daily peaks in mood, one relatively early in the morning and another near midnight.
Cornell sociologists Scott Golder and Michael Macey said this suggested mood was predominantly shaped by work-related stress.
Positive tweets were in abundance on Saturdays and Sundays, with morning tweets occurring two hours later in the day, which implied people slept later on weekends.
Bad moods peaked in the evening, which led researchers to believe people could be "emotionally refreshed" by sleep.
Researchers used language-monitoring software to analyse the tweets. The software contained lists of words, word stems and emoticons that corresponded to 64 positive and negative behavioural and psychological dimensions.