[en] We consider a free massive particle inside a box that is dragged by Rindler observers. Admitting that the particle obeys the Klein–Gordon equation, we find the frequencies of the stationary states of this system. Transitions between the stationary states are employed to set a standard frequency for a toy atomic clock. Comparing the energy spectrum of the accelerated system with the energy spectrum of an identical system in an inertial frame, we determine the influence of instantaneous acceleration on the rate of atomic clocks. We argue that our result does not violate the clock hypothesis. (note)