The principle of light therapy is to start from the retina. After receiving light stimulation, the ganglion cells of the retina are connected to the suprachiasmatic nucleus of the hypothalamus through the nerve pathway Retinohypothalamic tract (the nerve connection between the ganglion cells and the hypothalamus). After that, the signal is sent to the pineal gland to inhibit the production of melatonin. Melatonin is the physiological clock in charge of the human body. During the day, there is light stimulation, which inhibits the concentration of melatonin. At night, the light stimulation becomes weaker and inhibits The effect becomes worse and the concentration of melatonin rises. The most suitable and sleepy time is when the concentration of melatonin in the body is the highest. Therefore, sleep regulation is mainly because light therapy can change the concentration of melatonin at different times.