Computer scientist Alan Mislove at Northeastern University in Boston and his colleagues have found that these "tweets" suggest that the west coast is happier than the east coast.
Also, the country seems happiest on Sundays but saddest on Thursdays.
300 million tweets were analysed, each of which was awarded a mood score based on the number of positive or negative words it contained.
For example, 'diamond', 'love' and 'paradise' indicate happiness, whereas 'funeral', 'rape' and 'suicide' are negative. 'Dentist' is fairly neutral.
A calculation of scores hour by hour created a timed series of mood maps.
"The visualisations are amazing and I think it is absolutely fascinating to see the nation's mood vary in near-real time," New Scientist quoted Johan Bollen of Indiana University in Bloomington, as saying.
He, however, thinks there are certain hitches in the system. For instance, if someone types tweets 'I am not happy', the system counts the tweet as positive because of the word 'happy'.
Plus, the massive volume of data means the subtleties are lost.
But Twitter being available publicly, polling becomes quicker, cheaper and simpler.
"Twitter offers researchers a unique, live data set that changes by the minute," said Steven Gray at University College London.