Proper physical activity, even at a very light intensity such as walking or slow running, improves brain health related to prefrontal executive function and hippocampal memory. However, the neural mechanism behind the cognitive enhancement that occurs during dynamic aerobic exercise is elusive and remains unclear in humans. Recently, pupillometry has been attracting attention as a kind of readout of the brain’s ascending arousal mechanism, especially for brain noradrenergic and cholinergic system activation. Thus, to identify the neural mechanism behind the effects of very-light-intensity exercise, our recent work has focused on pupillometry during aerobic exercise, and we have successfully shown the efficacy of pupil dilation as a biological marker, even during very-light-/light-intensity exercise (below the ventilatory threshold). Interestingly, neuromelanin-MRI contrast in the LC, a marker of LC integrity, predicted the magnitude of exercise-induced pupil dilation and psychological arousal changes at the individual level. In addition, we have found that pupil dilation during exercise predicted the positive impact of acute very-light-/light-intensity exercise on prefrontal executive performance and hippocampal memory performance. The series of exercise pupillometry studies we will discuss here provides essential insights into the neural substrates of the advantages of exercise-induced brain stimulation in humans.