Reflecting on previous activities can improve episodic memory and well-being. While manually recording activities and experiences is not always feasible, there is a range of mobile sensors that allow implicit recording of users' lives. In this work, we investigate how reflection on daily summaries created from implicitly collected data improves episodic memory. Therefore, we built ReflectiveDiary, an Android application that collects personal data streams to create daily summaries automatically. Over the course of 16 days, we collected data from 11 participants using information, such as calls, messages, and calendar data to help people recollect their activities. By comparing reflected with non-reflected days, we show that reflecting on implicitly collected data improves remembering of events and their surrounding details. We further present an analysis of different types of memory cues and their usefulness to inform the design of reflective tools that help people to improve their episodic memory.