In this paper, we study two large data sets containing the information of two different human behaviors: blog-posting and wiki-revising. In both cases, the interevent time distributions decay as power laws at both individual and population level. Unlike previous studies, we put emphasis on time scales and obtain heterogeneous decay exponents in the intra- and inter-day range for the same dataset. Moreover, we observe opposite trend of exponents in relation to individual Activity. Further investigations show that the presence of intra-day activities mask the correlation between consecutive inter-day activities and lead to an underestimate of Memory, which explain the contradicting results in recent empirical studies. Removal of data in intra-day range reveals the high values of Memory and lead us to convergent results between wiki-revising and blog-posting. Copyright © 2011 Europhysics Letters Association.