  We consider a free massive particle inside a box which 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 the instantaneous acceleration on the rate of atomic clocks. We
argue that our result does not violate the clock hypothesis.
