Figure 2. 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 (see Table 1) 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 (see Table 2) 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.