Significance Light is an important regulator of endogenous circadian clocks. Regular light–dark cycles set circadian phase to ensure synchrony with external time, while irregular or inappropriate light exposure can disrupt circadian rhythms. We ask here whether light can also impact another key parameter of circadian rhythms, their amplitude, under conditions of stable entrainment. Using a diurnal rodent species and carefully calibrated light stimuli, we show that increasing the intensity of daytime light enhances the reproducibility and robustness of behavioral and physiological rhythms and increases the amplitude of circadian rhythms in electrical activity in the central brain pacemaker. These findings reveal an impact of light on circadian amplitude and highlight the potential importance of daytime light exposure for circadian health.