Recent high-resolution numerical simulations have shown that the diurnal variability in the atmospheric forcing strongly affects the dynamics, stability, and turbulence of submesoscale structures in the surface boundary layer (SBL). Field observations supporting the real-ocean relevance of these studies are, however, largely lacking at the moment. Here, the impact of large diurnal variations in the surface heat flux on a dense submesoscale upwelling filament in the Benguela upwelling system is investigated, based on a combination of densely-spaced turbulence microstructure observations and surface drifter data. Our data show that during nighttime and early-morning conditions, when solar radiation is still weak, frontal turbulence is generated by a mix of symmetric and shear instability. In this situation, turbulent diapycnal mixing is approximately balanced by frontal restratification associated with the cross-front secondary circulation. During daytime, when solar radiation is close to its peak value, the SBL quickly restratifies, the conditions for frontal instability are no longer fulfilled, and SBL turbulence collapses except for a thin wind-driven layer near the surface. The drifter data suggest that inertial oscillations periodically modulate the stability characteristics and energetics of the submesoscale fronts bounding the filament.