Daily Rhythms in Mobile Telephone Communication

Abstract

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.

Fig 1. The daily call patterns of 8 individuals (a-h).

The red lines denote the average fraction of calls placed at the corresponding hour for each of the three intervals I ₁ (solid line), I ₂ (dashed line), and I ₃ (dash-dotted line). The black line is the average call pattern of all 24 individuals over all intervals. Areas shaded green show where an individual’s fraction of calls exceeds the average, while areas shaded red show where it falls below the average.

Fig 2. Histogram of d _self and d _ref calculated for each ego.

This plot shows the results for all egos and all time intervals.

Fig 3. The relative entropies for the same 8 individuals as in Fig 1, calculated for 6-hour intervals (M: morning 6AM-12AM, A: afternoon 12AM-6PM, E: evening 6PM-0AM, N: night 0AM-6AM).

(ο): interval I ₁, (◻): interval I ₂, (◊): interval I ₃. The black line indicates the average relative entropy for all 24 individuals over all three intervals.

Fig 4. The fractions of calls to the two top-ranked alters for the same 8 individuals as in Fig 1, calculated for the same 6-hour intervals as in Fig 3 (M, A, E, N).

(ο): interval I ₁, (◻): interval I ₂, (◊): interval I ₃.

Fig 5. Average duration of calls made by males and females to their kin, friends, and all social contacts.

Fig 6. Comparison of the average duration of calls to social contacts of the same and opposite gender, separately for females and males.