Quantifying human circadian pacemaker response to brief, extended, and repeated light stimuli over the phototopic range

The authors' previous models have been able to describe accurately the effe cts of extended (similar to 5 h) bright-light (>4000 lux) stimuli on the ph ase and amplitude of the human circadian pacemaker, but they are not suffic ient to represent the surprising human sensitivity to both brief pulses of bright light and light of more moderate intensities. Therefore, the authors have devised a new model in which a dynamic stimulus processor (Process L) intervenes between the light stimuli and the traditional representation of the circadian pacemaker as a self-sustaining Limit-cycle oscillator (Proce ss P). The overall model incorporating Process L and Process P is intended to allow the prediction of phase shifts to photic stimuli of any temporal p attern (extended and brief light episodes) and any Light intensity in the p hotopic range. Two time constants emerge in the Process L model: the charac teristic duration for necessary bright-light pulses to achieve their full e ffect (5-10 min) and the characteristic stimulus-free (dark) interval that can be tolerated without incurring an excessive penalty in phase shifting ( 30-80 min). The effect of reducing light intensity is incorporated in Proce ss L as an extension of the time necessary for the light pulse to be fully realized (a power-law relation between time and intensity). This new model generates a number of new testable hypotheses, including the surprising pre diction that 24-h cycles consisting of 8 h of darkness and 16 h of only sim ilar to 3.5 lux would be capable of entraining a large fraction of the adul t population (similar to 45%). Experimental data on the response of the hum an circadian system to lower light intensities and briefer stimuli are need ed to allow for further refinement and validation of the model proposed her e.