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

1. Ocular exposure to early morning room light can significantly advance th e timing of the human circadian pacemaker. The resetting response to such l ight has a non-linear relationship to illuminance. The dose-response relati onship of thr: human circadian pacemaker to late evening light of dim to mo derate intensity has not been TI ell established. 2. Twenty-three healthy young male and female volunteers took part in a 9 d ay 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 exposur e on the endogenous circadian phase of the melatonin rhythm and the acute e ffects of the light exposure on plasma melatonin concentration were calcula ted. 3. We demonstrate that humans are highly responsive to the phase-delaying e ffects of light during the early biological night and that both the phase r esetting response to light and the acute suppressive effects of light on pl asma 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-delayin g 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 ligh t (dim room light of similar to 100 lx). The same held true fur the acute s uppressive effects of light on plasma melatonin concentrations. This indica tes that even small changes in ordinary light exposure during the late even ing hours can significantly affect both plasma melatonin concentrations and the entrained phase of the human circadian pacemaker.