Abstract: A characteristic (e.g., SpO2) of a user's physiological signals can be estimated using background sensing with a pulse oximeter. For example, measurements by the optical sensor may be made when one or more criteria are satisfied that indicate conditions for a reliable SpO2 reading. In some examples, the measurements by the optical sensor can be spaced out in time. In some examples, a background measurement by the optical sensor can include multiple windows of data that can be processed, and the results of processing can be combined to determine the SpO2 estimate. In some examples, a background measurement can be terminated earlier and/or processing of some windows of data can be skipped when sufficient physiological signals have been collected to generate an estimate of SpO2 or when the conditions during the measurement(s) indicate that the physiological signals are unlikely to yield a reliable estimate of SpO2.