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 …