Sensitivity of the human circadian pacemaker to nocturnal light: melatonin phase resetting and suppression

1. Ocular exposure to early morning room light can significantly advance the timing of the human circadian pacemaker. The resetting response to such light has a non-linear relationship to illuminance. The dose-response relationship of thr: human circadian pacemaker to late evening light of dim to moderate intensity has not been TI ell established. 2. Twenty-three healthy young male and female volunteers took part in a 9 day protocol in which a single experimental light exposure 6.5 h in duration nias given in the early biological night. The effects of the light exposure on the endogenous circadian phase of the melatonin rhythm and the acute effects of the light exposure on plasma melatonin concentration were calculated. 3. We demonstrate that humans are highly responsive to the phase-delaying effects of light during the early biological night and that both the phase resetting response to light and the acute suppressive effects of light on plasma melatonin follow a logistic dose-response curve, as do many circadian responses to light ill mammals. 4.Contrary to expectations, we found that half of the maximal phase-delaying response achieved in response to a single episode of Evening bright light (similar to 9000 lux (Ix)) carl be obtained with just over 1% of this light (dim room light of similar to 100 lx). The same held true fur the acute suppressive effects of light on plasma melatonin concentrations. This indicates that even small changes in ordinary light exposure during the late evening hours can significantly affect both plasma melatonin concentrations and the entrained phase of the human circadian pacemaker.