  Circadian rhythms are known to be important drivers of human activity and the
recent availability of electronic records of human behaviour has provided
fine-grained data of temporal patterns of activity on a large scale. Further,
questionnaire studies have identified important individual differences in
circadian rhythms, with people broadly categorised into morning-like or
evening-like individuals. However, little is known about the social aspects of
these circadian rhythms, or how they vary across individuals. In this study we
use a unique 18-month dataset that combines mobile phone calls and
questionnaire data to examine individual differences in the daily rhythms of
mobile phone activity. We demonstrate clear individual differences in daily
patterns of phone calls, and show that these individual differences are
persistent despite a high degree of turnover in the individuals' social
networks. Further, women's calls were longer than men's calls, especially
during the evening and at night, and these calls were typically focused on a
small number of emotionally intense relationships. These results demonstrate
that individual differences in circadian rhythms are not just related to broad
patterns of morningness and eveningness, but have a strong social component, in
directing phone calls to specific individuals at specific times of day.
