  Multi-layered networks represent a major advance in the description of
natural complex systems, and their study has shed light on new physical
phenomena. Despite its importance, however, the role of the temporal dimension
in their structure and function has not been investigated in much detail so
far. Here we study the temporal correlations between layers exhibited by real
social multiplex networks. At a basic level, the presence of such correlations
implies a certain degree of predictability in the contact pattern, as we
quantify by an extension of the entropy and mutual information analyses
proposed for the single-layer case. At a different level, we demonstrate that
temporal correlations are a signature of a 'multitasking' behavior of network
agents, characterized by a higher level of switching between different social
activities than expected in a uncorrelated pattern. Moreover, temporal
correlations significantly affect the dynamics of coupled epidemic processes
unfolding on the network. Our work opens the way for the systematic study of
temporal multiplex networks and we anticipate it will be of interest to
researchers in a broad array of fields.
