Seasonal Affective Disorder(SAD) Light Therapy Principles.

Seasonal Affective Disorder(SAD) Light Therapy Principles.

Light therapy is a common treatment for Seasonal Affective Disorder(SAD) .

The higher the light, the lower the melatonin.

Exposure to high light in the morning reduces melatonin and one will be more energetic.

At night, when the light is reduced, melatonin will rebound and people will want to sleep as a way to regulate body rhythms and SAD.

Studies have shown that melatonin secretion decreases during the onset of depression and increases after the symptoms are relieved, suggesting that melatonin and depression are inextricably linked.

The process of light entering the human eye is shown in Figure 1. After light is projected onto the retina, the signal is transmitted from the intrinsically photoreceptor retinal ganglion cells (ipRGC) to the suprachiasmatic nucleus (SCN), and the SCN connects to the pineal gland through its specific neural pathway to secrete melatonin [11]. The secretion of melatonin has a circadian rhythm, with less melatonin secretion during the day and a rapid increase after dark, peaking at 2-3 am. Melatonin enters the bloodstream and travels throughout the body to regulate biological rhythms and neuroendocrine secretion.

Figure 1 The non-visual channel of light entering the eye

Figure 2 The spectral sensitivity of photopic and scotopic vision and circadian rhythms

The melatonin inhibition rates of the same light with luminous intensity of 3,000 lx, 1,000 lx, 500 lx, 350 lx, and 200 lx were 71%, 67%, 44%, 38%, and 16%, respectively, after irradiating the healthy human body at midnight for 1 h. The stronger the luminous intensity, the more obvious the melatonin inhibition effect. The stronger the illumination, the more obvious the inhibition of melatonin; the weaker the light, the longer it takes to inhibit melatonin secretion; and even if the irradiation time is increased, the inhibitory effect of the high-intensity light cannot be achieved. 

Figure 3 Melatonin production under different light intensity

Data：WANG Danni<Research on mechanism of combination of special LED on reducing symptoms of depression>