  Timings of human activities are marked by circadian clocks which in turn are
entrained to different environmental signals. In an urban environment the
presence of artificial lighting and various social cues tend to disrupt the
natural entrainment with the sunlight. However, it is not completely understood
to what extent this is the case. Here we exploit the large-scale data analysis
techniques to study the mobile phone calling activity of people in large cities
to infer the dynamics of urban daily rhythms. From the calling patterns of
about 1,000,000 users spread over different cities but lying inside the same
time-zone, we show that the onset and termination of the calling activity
synchronizes with the east-west progression of the sun. We also find that the
onset and termination of the calling activity of users follows a yearly
dynamics, varying across seasons, and that its timings are entrained to solar
midnight. Furthermore, we show that the average mid-sleep time of people living
in urban areas depends on the age and gender of each cohort as a result of
biological and social factors.
