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J Clin Endocrinol Metab. 1999 Sep;84(9):3395-8.

Light affects morning salivary cortisol in humans.

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  • 1Dept. Hypothalamic Integration Mechanisms, Netherlands Institute for Brain Research, Amsterdam.


The effect of light on the morning-cortisol peak in humans was investigated in fourteen healthy men by exposing them to darkness and to light of 800 lux during a 1-h period on two subsequent mornings. In the early morning, we demonstrated a temporary increase of salivary cortisol levels after awakening, while light exposure resulted in a +/- 35% further increase in cortisol levels. Cortisol levels 20 and 40 min after waking were significantly higher during 800 lux exposure than during darkness. In order to investigate the time-dependency, the experiment was repeated in the late evening. In the evening, light had no effect on cortisol levels. These results demonstrate that light conditions in the early morning have a strong impact on the morning-cortisol peak, but that evening cortisol levels are unaffected by light. The possible role of the circadian pacemaker as mediator of the light effect on cortisol level is discussed.

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