RESUMO
Physiological and behavioral rhythms are governed by an endogenous circadian clock. In this paper are reported the studies on soldier's circadian of melatonin concentrations by means of electricity and light pulses presented to the popliteal region (behind the knee). The results showed that the phase of melatonin concentrations can be regulated by both the electricity pulses and the light pulses. A systematic relation was found between the timing of the electro-stimulating and the magnitude and direction of phase shifts, resulting in the generation of a phase response curve. The phase response curve displayed photic model. These findings have implications for the development of more effective treatments for sleep and circadian rhythm disorders.