A method of monitoring cardiac activity of a user is provided. Heart beat time data is detected by a PPG sensor (110) in a health sensor (100) worn by the user (1). Motion data is detected by a motion sensor (120) in the health sensor (100). The output data of the PPG sensor (110), which comprises heart beat time data, is analyzed to detect arrhythmia periods. The detected heart beat time data, the detected motion data and the detected arrhythmia are forwarded to an external device, where the data can be stored. The output data of the PPG sensor (110) comprising the heart beat time data, the detected motion data and the detected arrhythmia periods are displayed such that a medical professional (2) can analyze this data to determine whether a suspected arrhythmia period is valid.