Circadian clocks are usually modeled by limit cycle oscillators. These models allow theoretical analyses of the dynamical properties of circadian oscillations, including their entrainment by light-dark cycles and resetting by light pulses. These models account for the emergence of oscillations at the level of single cells. Recent single cell experiments however revealed a great cell-to-cell heterogeneity in the oscillatory behaviour. Theoretical studies and numerical simulations have shown that efficient synchronization can be achieved when individual oscillators are damped. An analytical investigation of the circadian Goodwin model further shows that damped oscillators can be entrained on larger ranges of periods of the light-dark cycle. Together, the experimental observations and theoretical results suggest that the limit-cycle-like oscillations result from the coupling between more sloppy (presumably damped) oscillators.