Previous research has shown that modified noun phrases (henceforth NPs) are subsequently retrieved faster than unmodified NPs. This effect is often called the “semantic complexity effect”. However, little is known about its mechanisms and underlying factors. In this study, we tested whether this effect is truly caused by the semantic information added by the modification, or whether it can be explained by the sheer amount of time that the processor spends expecting or maintaining an NP in the encoding phase. The results showed that time spent expecting or maintaining an NP can explain the effect over and above semantic and/or syntactic complexity. Our results challenge the current memory-based mechanisms for the modification effect such as the “distinctiveness” and “head-reactivation” accounts, and offer new and valuable insight into the memory processes during sentence comprehension.