A monitoring device configured to be attached to a subject includes a photoplethysmography (PPG) sensor configured to measure physiological information from the subject, and at least one processor configured to process signals from the PPG sensor to determine heart rate and RR-interval (RRi) for the subject, and to determine a heart rate pattern for the subject over a period of time. The at least one processor is configured to change a sampling frequency of the PPG sensor for determining RRi in response to the determined heart rate pattern. The at least one processor is configured to reduce the sampling frequency of the PPG sensor in response to determining a pattern of heart rate below a threshold.