The phase response curve is a mathematical method used to characterize changes in biological phenomena with temporal cycles, such as heartbeat and circadian rhythm. When exposed to white light for a certain period, the change in the circadian rhythm causes shifts (phase shift) in this curve. It’s called phase delay, which delays the urge to sleep. The opposite is called the phase advance, which speeds up sleep onset. The phase response curve is closely related to melatonin secretion in the brain.