2025ApJ...987..189E

IRAC data have long implied that early (z ≳ 6) galaxies often have very high specific star formation rates (sSFRs ≳ 30 Gyr^–1), but JWST data have shown that at least some early galaxies are forming stars far less vigorously. Here, we systematically analyze the recent star formation histories (SFHs) of a large (N = 368) sample of z ∼ 6 Lyman-break galaxies (LBGs) spanning -22 ≲ M_UV ≲ -16 assembled from Advanced Camera for Surveys+NIRCam imaging in the GOODS and A2744 fields. We find that very low Hα-to-UV luminosity ratios (L_Hα/L_UV) and strong recent downturns in star formation rate (SFR) are ≈5× more common among the UV-faintest subset of our sample (〈M_UV〉 = -17.4) compared to the brightest subset (〈M_UV〉 = -20.0). The frequency of high L_Hα/L_UV and strong recent SFR upturns is approximately constant with UV luminosity. We discuss how bursty SFHs naturally reproduce this much greater diversity in recent SFHs among very UV-faint galaxies. Using public NIRSpec/prism data, we newly confirm recent strong SFR downturns among three LBGs in our sample, and validate our photometric inferences on key SFH signatures among z ∼ 6 LBGs in general. Our results imply that early galaxies frequently cycle through phases of rapid stellar mass assembly and other periods of much slower growth. This yields huge (≳1-2 mag) fluctuations in M_UV on rapid (∼10-30 Myr) timescales, helping explain the surprising abundance of z > 10 galaxies. Finally, we caution that this burstiness causes all existing high-redshift samples (particularly line-selected samples) to be far less complete to galaxies with long recent phases of low sSFR compared to those currently undergoing a burst.