Duncan Watts at Yahoo Research in New York City and a few pals studied the time of day at which around 3000 individuals at a European university sent emails over an 83-day period as well as the email habits of over 122,000 e-mailers at a US university over a 2-year period.
They found two distinct types of emailer. They termed the first “day labourers” because they tended to send emails throughout the normal working day between 0900 and 1800 but not at other times. The second group they called “emailaholics” because these people sent emails throughout the waking hours from 0900 to 0100.
I tried to have a look at the original paper, but since this story has been slashdotted, I suspect the server on which it resides has turned into a burned out cinder. The methods used to produce this analysis would be very interesting to look at. It looks a little too clean to me!