Disclosures of the present invention describe a method for inhibiting melatoninsecretion. In the present invention, an illumination device is particularly configured for emitting an illumination light with a specific wavelength that is in a range between 58 5 1nm and 590nm, and a variety of experimental data have proved that the illumination light functions as a melatonin secretion inhibitor. Therefore, the illumination device of the present invention is evidenced to be suitable for use in light therapy, so as to treat seasonal affective disorder (SAD) by way of inhibiting melatonin secretion. Briefly speaking, after a SAD patient receives the irradiation 10 of the illumination light having wavelength ranged in 581-590 nm for a treatment course, the SAD patient is eventually leaded to stay in good spirits and a stable condition of emotion, thereby curing the abnormal symptoms of sleepiness and emotional disorder. |