An illustrative example system includes at least one alertness detector that is configured to detect an alertness condition of a driver of a vehicle and an alertness condition of a passenger in the vehicle. A controller is configured to determine a sleep risk factor based on the alertness condition of the passenger. The controller is also configured to determine a likelihood that the driver is sleepy based on the alertness condition of the driver and the sleep risk factor. The controller is configured to control a feature of the vehicle to assist the driver when the determined likelihood satisfies a predetermined criterion.