1. A method for determining a vigilance state of a vehicle driver, comprising: generating a noise signal; applying a supplementary torque to a steering assembly as a function of the noise signal; determining a reaction signal of the vehicle driver; determining at least one parameter of a parameterized transfer function from the noise signal to the reaction signal; and determining the vigilance state (a) from a time characteristic of the noise signal and a time characteristic of the reaction signal and (b) as a function of at least one of (i) an absolute value of the at least one parameter and/or (ii) a relative change of the at least one parameter with regard to at least one earlier parameter determined at an earlier instant in time; wherein the parameterized transfer function includes a linear transfer function and/or time-invariant transfer function.