This thesis investigates star formation during the first two billion years of cosmic history using James Webb Space Telescope observations. I present the FRESCO spectroscopic data release, providing emission-line measurements and redshifts across 0 < z < 9 and revealing galaxy overdensities at z > 5. Combining FRESCO and CONGRESS observations, I derive the first spectroscopic Hα luminosity functions at z ∼ 4–6.5 and measure the cosmic star formation rate density. The results show declining star formation activity towards higher redshift, consistent with ultraviolet-based estimates. Finally, a systematic search through archival NIRSpec spectroscopy identifies 14 dormant galaxies at 5 < z < 7.4, selected through weak Hα emission and strong Balmer breaks. Their inferred histories indicate that star formation ceased approximately 10–25 million years before observation. Together, these findings connect population-wide trends with intermittent stellar growth in individual galaxies, highlighting the importance of bursty star formation in the early Universe.