To understand light-induced immune modulation in vivo, we evaluated the effect of nocturnal light exposure on splenic natural killer (NK) cell activity in rats. Nocturnal increases in the number and cytotoxic activity of NK cells were significantly suppressed under extended light exposure. Nocturnal increases in serum corticosterone levels were also suppressed by light exposure, suggesting that the light exposure-induced decrease in NK cell functions was not due to stress-induced glucocorticoid secretion in rats. These results suggest that nocturnal light exposure could profoundly affect NK cell functions perhaps through eye-brain hormonal modulation.
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