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, controlled by sunrise and sunset, reports the New York Times.

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.