  Light scalar Dark Matter with scalar couplings to matter is expected within
several scenarios to induce variations in the fundamental constants of nature.
Such variations can be searched for, among other ways, via atomic spectroscopy.
Sensitive atomic observables arise primarily due to possible changes in the
fine-structure constant or the electron mass. Most of the searches to date have
focused on slow variations of the constants (i.e. modulation frequencies $<$ 1
Hz). In a recent experiment \mbox{[Phys. Rev. Lett. 123, 141102 (2019)]} called
WReSL (Weekend Relaxion-Search Laboratory), we reported on a direct search for
rapid variations in the radio-frequency band. Such a search is particularly
motivated within a class of relaxion Dark Matter models. We discuss the WReSL
experiment, report on progress towards improved measurements of rapid
fundamental constant variations, and discuss the planned extension of the work
to molecules, in which rapid variations of the nuclear mass can be sensitively
searched for.
