  The temporal statistics exhibited by written correspondence appear to be
media dependent, with features which have so far proven difficult to
characterize. We explain the origin of these difficulties by disentangling the
role of spontaneous activity from decision-based prioritizing processes in
human dynamics, clocking all waiting times through each agent's `proper time'
measured by activity. This unveils the same fundamental patterns in written
communication across all media (letters, email, sms), with response times
displaying truncated power-law behavior and average exponents near -3/2. When
standard time is used, the response time probabilities are theoretically
predicted to exhibit a bi-modal character, which is empirically borne out by
our new years-long data on email. These novel perspectives on the temporal
dynamics of human correspondence should aid in the analysis of interaction
phenomena in general, including resource management, optimal pricing and
routing, information sharing, emergency handling.
