Portion of the Phase Response Curve (PRC) during which light has no effect on rhythm phase. There are two major parts in the PRC derived from tests with light pulses. During the “subjective night,” light induces phase shifts (except at the “crossover” point), but during most of the “subjective day,” circadian rhythm phase is generally not modified by light. This portion of the PRC during which light has no effect on rhythm phase is known as the “dead zone.”

 Circadian Rhythm

 Phase Response Curve (PRC)