Light is the most important environmental signal for synchronizing human circadian ('about a day') rhythms. The circadian system is normally synchronized with the solar day, ensuring that the sleep-wake cycle and endocrine rhythms are timed appropriately; however, electrical lighting can also be used to reset circadian rhythms. In this short review, we discuss the properties of light stimuli that are important for resetting the human circadian system. The direction and magnitude of light resetting are circadian phase-dependent, with exposure to light in the early biological night resulting in a phase-delay shift of circadian rhythms, and exposure to light in the late biological night and early morning resulting in a phase advance shift. Circadian resetting responses can be enhanced by increasing the irradiance or duration of the light stimulus, or by using short-wavelength blue light to activate intrinsically photosensitive retinal ganglion cells that express the photopigment melanopsin. This knowledge can potentially be applied to improve light therapy for circadian rhythm sleep disorders, and to help reset circadian rhythms in individuals exposed to shift work or jet lag.
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