Alertness-enhancing effects of bright light are particularly strong at night or after sleep deprivation. Alerting effects during daytime also exist, yet these appear to be more modest. In this study, we investigate whether a higher illuminance level particularly benefits individuals who suffer from mental fatigue e not from sleep pressure, but from mental exertion. A 2 2 within-subjects design (N 1/4 28; 106 sessions) was applied to investigate effects of 1000 vs. 200 lx at the eye on self-report measures, task performance and physiological arousal after a mental antecedent condition (fatigue vs. control). Results showed that participants felt less sleepy, more vital and happier when exposed to bright light. Effects on subjective sleepiness and self-control capacity were stronger under mental fatigue. Vigilance benefited from bright light exposure e although this effect emerged with a delay irrespective of the antecedent condition. Other tasks showed more mixed and sometimes even adverse effects of bright light.