  Patterns of deliberate human activity and behavior are of utmost importance
in areas as diverse as disease spread, resource allocation, and emergency
response. Because of its widespread availability and use, e-mail correspondence
provides an attractive proxy for studying human activity. Recently, it was
reported that the probability density for the inter-event time $\tau$ between
consecutively sent e-mails decays asymptotically as $\tau^{-\alpha}$, with
$\alpha \approx 1$. The slower than exponential decay of the inter-event time
distribution suggests that deliberate human activity is inherently
non-Poissonian. Here, we demonstrate that the approximate power-law scaling of
the inter-event time distribution is a consequence of circadian and weekly
cycles of human activity. We propose a cascading non-homogeneous Poisson
process which explicitly integrates these periodic patterns in activity with an
individual's tendency to continue participating in an activity. Using standard
statistical techniques, we show that our model is consistent with the empirical
data. Our findings may also provide insight into the origins of heavy-tailed
distributions in other complex systems.
