A monitoring device configured to be attached to a subject includes a photoplethysmography (PPG) sensor configured to detect/measure physiological information from the subject, and a processor configured to process the physiological information to detect subject stress, and to determine an origin of the subject stress. The processor can determine the origin of the subject stress by increasing a sampling rate of the PPG sensor to collect higher acuity physiological information. The processor also can determine the origin of the subject stress by processing data from the PPG sensor to determine whether the subject is likely to have atrial fibrillation. In response to determining that the subject is likely to have atrial fibrillation, the processor can increase a frequency of pulsing of an optical emitter of the PPG sensor and/or increase a sampling rate of the PPG sensor to collect higher acuity data for diagnosing that atrial fibrillation is truly occurring.