A user-wearable device includes a front facing first light detector and a backside optical sensor, which faces the user's 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 user's 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 user's 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.