1. A method for ascertaining a stress level of a driver of a vehicle, comprising: determining one of a setpoint trajectory of the vehicle and a state variable correlating with the setpoint trajectory; comparing the one of the setpoint trajectory and the state variable respectively to at least one of an actual trajectory and a state variable correlating with the actual trajectory; inferring a higher stress level from at least one of an increasing frequency and a magnitude of a deviation of one of the actual trajectory from the setpoint trajectory and the state variable correlating with the setpoint trajectory from the state variable correlating with the actual trajectory; and taking a stress-reducing measure as a function of the stress level.