Melatonin Phase Response describes the time-dependent change, or phase shift, in the body’s circadian rhythm that occurs following exposure to an external stimulus, most notably light or exogenous melatonin. The magnitude and direction of this shift are governed by the Phase Response Curve (PRC), which dictates that stimuli given during the early biological night cause a phase delay, while those given during the late biological night or early morning cause a phase advance. Understanding this response is fundamental to correcting circadian misalignment, such as jet lag or delayed sleep phase disorder.
Origin
This is a central concept in chronobiology, derived from the established principle of the Phase Response Curve, which maps the response of a biological oscillator to a stimulus across its cycle. ‘Melatonin’ is included because its secretion rhythm is the most commonly tracked output of the clock, and the phase shift is measured by changes in its onset or peak. This principle provides a predictive model for chronotherapy.
Mechanism
The phase response is mediated by the effect of the stimulus on the neurons of the Suprachiasmatic Nucleus (SCN). Light exposure in the early night inhibits SCN activity, effectively delaying the clock, while light in the morning stimulates the SCN, advancing the clock. Conversely, exogenous melatonin acts on MT1 and MT2 receptors in the SCN, mimicking the signal of darkness and causing a phase advance when administered in the late afternoon or early evening, and a phase delay in the morning.
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