The present invention relates toa 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).
