A processor-based personal electronic device (such as a smartphone) is programmed to automatically respond to data sent by various sensors from which the user's activity may be inferred. One or more alarms on the device may be temporarily disabled when sensor data indicates that the user is asleep. One or more of the sensors may be worn by the user and remote from the device. A wireless communication link may be used by the device to obtain remote sensor data. Data from on-board sensors in the device—such as motion sensors, location sensors, ambient light sensors, and the like—may also be used to deduce the user's current activity. User data (such as calendar entries) may also be used to determine likely user activity and set alarms accordingly. Biometric data from a second, nearby person may also be used to automatically select certain alarm modes on a first person's device.