Our experience of web access slowing down is a consequence of the aggregated web access pattern of web users. This is just one example among several human-oriented services which are strongly affected by human activity patterns. Recent empirical evidence is indicating that human activity patterns are characterized by power-law distributions of inter-event times, where large fluctuations rather than regularity is the common case. I show that this temporal heterogeneity can be explained by two mechanisms: (i) humans have some perception of their past activity rate and (ii) based on that they react by accelerating or reducing their activity rate. Using these two mechanisms I explain the inter-event time statistics of Darwin's and Einstein's correspondence and the email activity within an university environment. Moreover, they are typical examples of the accelerating and reducing class, respectively. These results are relevant to the system design of human-oriented services.
