  Human activity follows an approximately 24-hour day-night cycle, but there is
significant individual variation in awake and sleep times. Individuals with
circadian rhythms at the extremes can be categorized into two chronotypes:
"larks", those who wake up and go to sleep early, and "owls", those who stay up
and wake up late. It is well established that a person's chronotype can affect
their activities and health. However, less is known on the effects of
chronotypes on the social behavior, even though it is evident that social
interactions require coordinated timings. To study how chronotypes relate to
social behavior, we use data collected using a smartphone app on a population
of more than seven hundred volunteer students to simultaneously determine their
chronotypes and social network structure. We find that owls maintain larger
personal networks, albeit with less time spent per contact. On average, owls
are more central in the social network of students than larks, frequently
occupying the dense core of the network. Owls also display strong homophily, as
seen in an unexpectedly large number of social ties connecting owls to owls.
