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.
“By the end of the year, the pattern somehow comes back to the same point where it was one year ago,” said Daniel Monsivais, a doctoral candidate at the University of Helsinki and a co-author of the paper. The timing of this shift wasn’t random: It moved in tandem with the lengthening of days during summer and shorter days of winter.
Other factors might affect the times that people make phone calls, including school and work schedules. To factor out these social influences, the researchers ordered cities according to how far west they were in the time zone. They then looked to see whether a city with a slightly earlier sunrise and sunset saw a corresponding shift in its calling pattern, compared to a city with a later diurnal cycle.
Indeed, the timing of the last calls and the first calls closely tracked the movement of the sun. In one group of five cities, there was about a 40-minute difference between the easternmost city and the westernmost one, even though schools got out at the same time and other factors were the same.
Does the timing of sunrise and sunset somehow affect our circadian rhythms in a way that is visible in cellphone records? Call records do provide a reasonable estimate of when people are awake and asleep, other research has found, and the early morning lull in calls is around the midpoint of people’s time asleep.
If the midpoint of darkness moves over the course of the year — shifting earlier or later as the days change in length — maybe, they reason, melatonin production might change with the seasons, too. It could be further evidence that the chemicals that govern our body clocks are linked to events in the sky.
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