Light resetting and entrainment of human circadian rhythms

Light is the most important synchronizer of human circadian rhythms. In this chapter, we review the resetting effects of light on the human circadian system. First, we describe the importance of circadian entrainment and how the circadian system is organized to fulfill this purpose. Then, we discuss factors that influence the magnitude of circadian light responses, including the circadian phase of light exposure and characteristics of the light stimulus such as intensity, duration, and wavelength. Finally, we review the effects of electrical lighting and sunlight on the timing of circadian rhythms in laboratory studies and under real-world conditions. The main summary points of the chapter are: 1. Exposure to light in the early biological night induces a phase delay shift of human circadian rhythms, whereas exposure to light in the late biological night induces a phase advance shift. 2. Circadian responses to light can be enhanced by increasing the intensity or duration of the light stimulus, using short-wavelength light, or exposing oneself to dim light prior to the resetting stimulus. 3. The phase-resetting effects of light on circadian rhythms are greatest near the early part of a continuous light stimulus, as compared to the later part. 4. The human circadian system can be reset and entrained by exposure to electrical lighting including ordinary room light. 5. The circadian timing of sleep and other rhythms is modulated by exposure to electrical lighting and natural lighting under real-word conditions.