A method comprising: performing using one or more PPG sensor units, a first sequence of heart rate signal measurement, each heart rate signal measurement of the first sequence indicative of beat-to-beat timing of a series of heartbeats of a user; for each heart rate signal measurement of the first sequence: determining whether a variance of the beat-to-beat timing of the series of heartbeats is indicative of an irregular rhythm; in accordance with a determination that the variance is indicative of an irregular rhythm, increasing an occurrence value; determining whether the occurrence value meets an occurrence threshold; in accordance with the occurrence value meeting the occurrence threshold, registering an instance; in accordance with the occurrence value not meeting the occurrence threshold: wait a second amount of time; and determining whether a third amount of time has elapsed since the last measurement; and in accordance with a determination that the third amount of time has elapsed: determining , based on a number of heart rate signal measurements during a predetermined period, a first amount of time to wait before performing a second sequence of heart rate signal measurements; waiting the first amount of time; and performing the second sequence of heart rate signal measurements. performing using one or more PPG sensor units, a first sequence of heart rate signal measurement, each heart rate signal measurement of the first sequence indicative of beat-to-beat timing of a series of heartbeats of a user; for each heart rate signal measurement of the first sequence: determining whether a variance of the beat-to-beat timing of the series of heartbeats is indicative of an irregular rhythm; in accordance with a determination that the variance is indicative of an irregular rhythm, increasing an occurrence value; determining whether the occurrence value meets an occurrence threshold; in accordance with the occurrence value meeting the occurrence threshold, registering an instance; in accordance with the occurrence value not meeting the occurrence threshold: wait a second amount of time; and determining whether a third amount of time has elapsed since the last measurement; and in accordance with a determination that the third amount of time has elapsed: determining , based on a number of heart rate signal measurements during a predetermined period, a first amount of time to wait before performing a second sequence of heart rate signal measurements; waiting the first amount of time; and performing the second sequence of heart rate signal measurements. determining whether the occurrence value meets an occurrence threshold; in accordance with the occurrence value meeting the occurrence threshold, registering an instance; in accordance with the occurrence value not meeting the occurrence threshold: wait a second amount of time; and determining whether a third amount of time has elapsed since the last measurement; and in accordance with a determination that the third amount of time has elapsed: determining , based on a number of heart rate signal measurements during a predetermined period, a first amount of time to wait before performing a second sequence of heart rate signal measurements; waiting the first amount of time; and performing the second sequence of heart rate signal measurements. determining , based on a number of heart rate signal measurements during a predetermined period, a first amount of time to wait before performing a second sequence of heart rate signal measurements; waiting the first amount of time; and performing the second sequence of heart rate signal measurements. <EOS>