Greater sensitivity of the circadian system of women to bright light, but not dim-to-moderate light

Abstract

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.

Keywords: evening light; gender differences; hormones; light sensitivity; melatonin suppression; menstrual phase; sex differences.

Figure 1

Melatonin levels during the 400‐lux and 2000‐lux conditions in men versus women. Average melatonin levels are plotted (mean ± SEM) for the women (orange) and men (blue) at each hourly time point during the (A) 400‐lux and (B) 2000‐lux light conditions. Significance is shown for independent samples t‐tests (*p < .05, †p < .10).

Figure 2

Percentage melatonin suppression under each light condition for men versus women. Suppression values are given as a percentage relative to dim light <1 lux conditions. Individual data points are shown as crosses for the women (orange) and men (blue). Group‐level dose response curves for women and men are plotted as thick orange and blue lines, respectively. Individual‐level dose–response curves are plotted as thin lines. A logarithmic x‐axis is used.

Figure 3

Percentage melatonin suppression under each light condition for luteal versus follicular phases in women. Suppression values are given as a percentage relative to dim light (<1 lux) conditions. Individual data points are shown as crosses for the luteal (pink) and follicular phases (blue). Group‐level dose response curves for luteal and follicular phases are plotted as thick pink and blue lines, respectively. The group‐level fit for all of the women (combining menstrual phases) is shown for reference as the think orange line. A logarithmic x‐axis is used.