Effect of Light on Human Circadian Physiology

The circadian system in animals and humans, being near but not exactly 24 hours in cycle length, must be reset on a daily basis to remain in synchrony with external environmental time. This process of entrainment is achieved in most mammals through regular exposure to light and darkness. In this article, we review the results of studies over the past 25 years conducted in our laboratory and in those of others investigating the effects of light on the human circadian timing system. These studies have revealed how the timing, intensity, duration, and wavelength of light affect the human biologic clock. Our most recent studies also demonstrate that much is yet to be learned about the effects of light on the human circadian timing system.