In this study, 12 healthy males were exposed to various light conditions (2300 K, 3000 K, 5000 K and dim) for 1.5 h at midnight. The conditions of 3000 K and 5000 K were created by commercial fluorescent lamps. The light at 2300 K was achieved by fitting a 3000 K fluorescent lamp with a special filter that absorbed short-wavelength light. The vertical illuminance level was kept at 200 lx. Saliva samples were taken before and after the light exposure. The light at 5000 K suppressed melatonin secretion acutely. The 2300 K lamp condition appeared to have no effect on melatonin secretion as well as the dim condition, while melatonin secretion was measurably suppressed by the light at 3000 K.
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