Women typically sleep and wake earlier than men and have been shown to have earlier circadian timing relative to the light/dark cycle that synchronizes the clock. A potential mechanism for earlier timing in women is an altered response of the circadian system to evening light. We characterized individual-level dose–response curves for light-induced melatonin suppression using a within-subjects protocol. Fifty-six participants (29 women, 27 men; aged 18–30 years) were exposed to a range of light illuminances (10, 30, 50, 100, 200, 400, and 2000 lux) using melatonin suppression relative to a dim control (<1 lux) as a marker of light sensitivity. Women were free from hormonal contraception. To examine the potential influence of sex hormones, estradiol and progesterone was examined in women and testosterone was examined in a subset of men. Menstrual phase was monitored using self-reports and estradiol and progesterone levels. Women exhibited significantly greater melatonin suppression than men under the 400-lux and 2000-lux conditions, but not under lower light conditions (10–200 lux). Light sensitivity did not differ by menstrual phase, nor was it associated with levels of estradiol, progesterone, or testosterone, suggesting the sex differences in light sensitivity were not acutely driven by circulating levels of sex hormones. These results suggest that sex differences in circadian timing are not due to differences in the response to dim/moderate light exposures typically experienced in the evening. The finding of increased bright light sensitivity in women suggests that sex differences in circadian timing could plausibly instead be driven by a greater sensitivity to phase-advancing effects of bright morning light.

中文翻译：

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女性的昼夜节律系统对强光更敏感，但对暗光至中等光不敏感

女性通常比男性更早入睡和起床，并且已被证明相对于同步时钟的光/暗周期具有更早的昼夜节律。女性提前计时的一个潜在机制是昼夜节律系统对傍晚光线的反应改变。我们使用受试者内方案描述了光诱导褪黑激素抑制的个体水平剂量反应曲线。 56 名参与者（29 名女性，27 名男性；年龄 18-30 岁）暴露于一系列光照度（10、30、50、100、200、400 和 2000 lux）下，使用相对于昏暗对照的褪黑激素抑制(<1 lux) 作为光敏感度的标志。妇女无需激素避孕。为了检查性激素的潜在影响，对女性进行了雌二醇和黄体酮的检查，并对一部分男性进行了睾酮的检查。使用自我报告以及雌二醇和黄体酮水平监测月经期。在 400 勒克斯和 2000 勒克斯条件下，女性比男性表现出明显更强的褪黑激素抑制作用，但在较低光照条件下（10-200 勒克斯）则不然。光敏感性不因月经阶段而异，也与雌二醇、黄体酮或睾酮水平无关，这表明光敏感性的性别差异并不是由性激素循环水平严重驱动的。这些结果表明，昼夜节律的性别差异并不是由于对晚上通常经历的微弱/中等光线照射的反应差异造成的。女性对亮光敏感度增加的发现表明，昼夜节律时间的性别差异可能是由于对明亮晨光的相位提前效应更敏感而驱动的。