Adult male Syrian hamsters maintained under 6-h light, 18-h dark cycles (lights out daily from 1200–0600 h) were exposed to either 1 or 5 sec light either 8 h (at 2000 h) or 12 h (at 2400 h) into the dark phase. The light had an irradiance of 32,000 μW/cm2. With both light pulse durations and at both times, melatonin levels were depressed to daytime values 30 min after the onset of the light pulse. Whereas pineal melatonin production eventually increased to high nighttime values in hamsters exposed to 1 sec light at either 2000 or 2400 h and in animals receiving a 5-sec light pulse at 2000 h, when the 5-sec light occurred at 2400 h, pineal melatonin levels remained low for the remainder of the night. Thus, both the placement and the duration of light exposure appear to be important in determining the ability of light to depress melatonin production in the Syrian hamster.
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