A user-wearable device includes a front facing first light detector and a backside optical sensor, which faces the users skin and includes a light source and a second light detector. The device also includes a skin tone detector and an ultraviolet (UV) exposure detector. The UV exposure detector is adapted to determine estimate(s) of a users exposure to UV light in dependence on signal(s) produced using the first light detector, calibrate UV exposure threshold(s) in dependence on a skin tone metric produced using the skin tone detector, compare estimate(s) of a users exposure to UV light to calibrated UV exposure threshold(s), and selectively trigger an alert in dependence on results of the comparison(s). The second light detector is also used to produce a photoplethysmography (PPG) signal from which measures heart rate (HR), heart rate variability (HRV), respiration rate (RR) or respiratory sinus arrhythmia (RSA) is/are produced.
