  Social media are transforming global communication and coordination and
provide unprecedented opportunities for studying socio-technical domains. Here
we study global dynamical patterns of communication on Twitter across many
scales. Underlying the observed patterns is both the diurnal rotation of the
earth, day and night, and the synchrony required for contingency of actions
between individuals. We find that urban areas show a cyclic contraction and
expansion that resembles heartbeats linked to social rather than natural
cycles. Different urban areas have characteristic signatures of daily
collective activities. We show that the differences detected are consistent
with a new emergent global synchrony that couples behavior in distant regions
across the world. Although local synchrony is the major force that shapes the
collective behavior in cities, a larger-scale synchronization is beginning to
occur.
