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Luís Amaral, associate professor of chemical and biological engineering at Northwestern University, made the surprising claim after studying emails sent and received from more than 3,000 accounts at a European university during a three-month period.

Amaral and his collaborators were inspired to create a mathematical model to investigate email behaviour after questioning papers that assumed people respond to emails in the most efficient way.

"I was not convinced, since I don't do it in a rational way," Amaral said.

The researchers revealed a model in which people send emails at random, but the probability of them sending emails during a given period depended on what that period was. If it was in the middle of the night, the probability was near zero. If it was during the weekend, the probability was much lower than during weekdays.

"The model explains all the data, and it shows that people have cycles in which they use certain services," Amaral said. "You can then make predictions based on those cycles to know when people are going to request a service. Even though it's random, there are peaks in demand that don't look random."