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With the successful launch and commissioning of JWST we are now able toroutinely spectroscopically probe the rest-frame optical emission of galaxiesat $z>6$ for the first time. Amongst the most useful spectral diagnostics usedin the optical is the Balmer/4000~\AA\ break; this is, in principle, adiagnostic of the mean ages of composite stellar populations. However, theBalmer break is also sensitive to the shape of the star formation history, thestellar (and gas) metallicity, the presence of nebular continuum emission, anddust attenuation. In this work we explore the origin of the Balmer/4000~\AA\break using the SYNTHESIZER synthetic observations package. We then makepredictions of the Balmer/4000~\AA\ break using the First Light andReionisation Epoch Simulations (FLARES) at $5<z<10$. We find that the averagebreak strength weakly correlates with stellar mass and rest-frame far-UVluminosity, but that this is predominantly driven by dust attenuation. We alsofind that break strength provides a weak diagnostic of the age but performsbetter as a means to constrain star formation and stellar mass, alongside theUV and optical luminosity, respectively.

First Light And Reionisation Epoch Simulations (FLARES) XIV: The Balmer/4000~\AA\ Breaks of Distant Galaxies