The heart beats at a minimum of 60 BPM (except in endurance athletes where the resting frequency can drop lower). One second of delay or more would offset the

The heart beats at a minimum of 60 BPM (except in endurance athletes where the resting frequency can drop lower). One second of delay or more would offset the flash after the next beat, and, since the heart frequency varies slightly spontaneously, it would probably prevent the phenomenon... or maybe not :-)