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. 2009 Apr 15;5(2):155–163.

Figure 3.

Figure 3

Illuminance-response curve of the human circadian pacemaker. The shift in the phase of the melatonin rhythm (A), as assessed on the day following exposure to a 6·5 h experimental light stimulus, has been fitted with a four parameter logistic model using a non-linear least squares analysis. Acute suppression of plasma melatonin (B) during the light exposure also has been fitted with a four parameter logistic model using a non-linear least squares analysis. The logistic models predict an inflection point of the curve (i.e. the sensitivity of the system) at ≈120 lx. Saturation of the phase-shift response is predicted to occur with ≈550 lx and saturation of the melatonin-suppression response is predicted to occur with ≈200 lx. Individual subjects are represented by ▪ the model by the continuous line, and the 95 % confidence intervals by the dotted lines. Reproduced from: Zeitzer JM, Dijk DJ, Kronauer RE, Brown EN, Czeisler CA. Sensitivity of the human circadian pacemaker to nocturnal light: melatonin phase resetting and suppression. J Physiol 2000; 526: 695-702 (with kind permission of Wiley-Blackwell, Oxford, UK).