The present invention relates to a system (34) for detecting variation of heart rate, HR, of a user (14), said system comprising an optical sensing unit (12) for measuring a heartbeat-related optical signal of said user (14) over time, a processing unit (20) for deriving an HR-variation signal from said heartbeat-related optical signal, an analyzing unit (22) for comparing said derived HR-variation signal to a reference HR range, and an initiating unit (26) for initiating a process for measuring an informative heartbeat-related signal of said user (14) depending on said comparing of said analyzing unit (22).
