Long after the sun has gone down, the electric lights keep blazing. That might suggest that most humans aren’t as influenced by Earth’s light-dark cycle as we used to be.
But a new study, drawing on the cellphone call records of more than a million people, shows that the times of day when they are active grew longer and shorter over the course of the year, waxing and waning with the daylight.
The new study, published on Tuesday in the journal PLOS Computational Biology, looked at city dwellers all living in the same time zone in southern Europe. In previous work with the same data, the researchers estimated how often users called one another.
Eventually, the scientists began to wonder whether there were patterns in the timing of calls.
As it turned out, there were clear peaks and dips in phone calls throughout the day. One peak in outgoing calls was always at midday, while another was in the evening. In one city the group studied, for example, the early peak was centered around noon, while another occurred at 9 p.m. The lowest likelihood of calls going out was at around 4 p.m. and 4 a.m.
Over the course of the year, however, there was a noticeable shift. The last call times crept later during a stretch of three or four months, even as the earliest call times grew earlier. The peak calling periods moved as well, with the morning peak moving earlier and the evening peak moving later. Then, the process reversed direction.