One or more sensors and/or sensing techniques are used individually or in combination to detect and/or measure heart rate and/or heart rate variability. The sensors may be wearable and/or portable and may operate on different principles and may include different power profiles. The sensors and/or sensing techniques may be used at the same time or at different times that may overlap partially or completely or may be mutually exclusive. A controller may select one or more sensors and/or sensing techniques based on operating conditions and/or based on conservation of power.
