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Dose-response relationships for resetting of human circadian clock by light


SINCE the first report in unicells1, studies across diverse species have demonstrated that light is a powerful synchronizer which resets, in an intensity-dependent manner, endogenous circadian pacemakers1–5. Although it is recognized that bright light (7,000 to 13,000 lux) is an effective circadian synchronizer in humans6–10, it is widely believed that the human circadian pacemaker is insensitive to ordinary indoor illumination (50–3001ux)11. It has been proposed that the relationship between the resetting effect of light and its intensity follows a compressive nonlinear function12, such that exposure to lower illuminances still exerts a robust effect13. We therefore undertook a series of experiments which support this hypothesis and report here that light of even relatively low intensity (180 lux) significantly phase-shifts the human circadian pacemaker. Our results clearly demonstrate that humans are much more sensitive to light than initially suspected and support the conclusion that they are not qualitatively different from other mammals in their mechanism of circadian entrainment14.

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Boivin, D., Duffy, J., Kronauer, R. et al. Dose-response relationships for resetting of human circadian clock by light. Nature 379, 540–542 (1996).

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