More and more so-called IoT-enhanced business processes (BPs) are supported by IoT devices, which collect large amounts of data about the execution of such processes. While these data have the potential to reveal crucial insights into the execution of the BPs, the absence of a suitable event log format integrating IoT data to process data greatly hampers the realisation of this potential. In this paper, we first discuss some recurrent challenges in the integration of IoT and process event data, from which we derive ten concrete requirements for a suitable IoT-enhanced event log format. Based on this, we present the Native Iot-Centric Event (NICE) log, a new event log format designed to incorporate IoT data into a process event log ensuring traceability and flexibility and limiting data loss. The new format was translated in an XML schema and linked to a smart spaces data simulator to generate synthetic logs. We evaluate our format against the ten requirements previously established for an IoT-enhanced event log format, showing that it meets all requirements, contrarily to other alternative formats. We then perform analyses of both a real-life event log and a synthetic log to show how IoT data can easily be used to explain anomalies in the process.