Kozaki, T., Hidaka, Y., Takakura, J. Y., & Kusano, Y. (2018). Suppression of salivary melatonin secretion under 100-Hz flickering and non-flickering blue light. Journal of physiological anthropology. https://doi.org/10.1186/s40101-018-0183-9
Kozaki T, Hidaka Y, Takakura JY, Kusano Y. Suppression of salivary melatonin secretion under 100-Hz flickering and non-flickering blue light. Journal of physiological anthropology. 2018. doi:10.1186/s40101-018-0183-9
Kozaki, Tomoaki, et al. "Suppression of salivary melatonin secretion under 100-Hz flickering and non-flickering blue light." Journal of physiological anthropology, 2018.
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RRM Academy Synopsis
Flickering and steady blue light both lowered melatonin in young men
Later in a 90-minute night exposure, both flickering and non-flickering blue light lowered melatonin. In this experiment, 11 young men sat under dim, 100-Hz flickering and non-flickering light. At 15 minutes, only non-flickering light had lowered melatonin significantly.
Key Findings
Twelve men aged 20 to 23 (mean ± S.D. = 21.6 ± 1.5 years) enrolled. One was excluded because his melatonin level fell below the assay's detection limit, leaving 11 men.
Melatonin levels differed significantly by lighting condition, and the pattern over time also depended on the condition (both p < 0.01).
Fifteen minutes into exposure (1:15 A.M.), non-flickering light had produced significantly lower melatonin than dim light. At that time point, 100-Hz flickering light and dim light showed no significant difference.
In the samples after 1:30 A.M., both blue light conditions, 100-Hz flickering and non-flickering, gave significantly lower average melatonin than dim light.
The suppression rate showed a trend toward a smaller drop under 100-Hz flickering than non-flickering light at 1:15 A.M. (p < 0.1). Later samples showed no significant gap between the two.
Interpretation
The study is a within-subject experiment. Each of 11 young Asian men sat through all three lighting conditions in random order, with sessions 6 days apart. Women took no part, and saliva melatonin was the only outcome measured. The authors conclude that 100-Hz flickering light may affect melatonin secretion about as much as non-flickering light does, and they describe the 15-minute difference as small and limited. They add that earlier work found a series of 2-ms flashes of bright light, one per minute for 60 minutes, delayed melatonin onset without changing melatonin secretion. By the authors' reasoning, a shift in body-clock timing under 100-Hz flickering light remains possible.
RRM Context
A 2014 review by Reiter and colleagues reports that melatonin in the ovarian follicle protects the egg from oxidative stress, especially around ovulation. The review also notes that light at night suppresses the high nighttime melatonin level. Restorative reproductive medicine treats the ovulatory cycle as a body process shaped by its surroundings. Light at night is one of them.
Our editorial summary of this paper, not the article's abstract.
Abstract
Background
Bright light at night is known to suppress melatonin secretion. Novel photoreceptors named intrinsically photosensitive retinal ganglion cells (ipRGCs) are mainly responsible for projecting dark/bright information to the suprachiasmatic nucleus and thus regulating the circadian system. However, it has been shown that the amplitude of the electroretinogram of ipRGCs is considerably lower under flickering light at 100 Hz than at 1-5 Hz, suggesting that flickering light may also affect the circadian system. Therefore, in this study, we evaluated light-induced melatonin suppression under flickering and non-flickering light.
Methods
Twelve male participants between the ages of 20 and 23 years (mean ± S.D. = 21.6 ± 1.5 years) were exposed to three light conditions (dim, 100-Hz flickering, and non-flickering blue light) from 1:00 A.M. to 2:30 A.M., and saliva samples were obtained just before 1:00 A.M. and at 1:15, 1:30, 2:00, and 2:30 A.M.
Results
A repeated measures t test with Bonferroni correction showed that at 1:15 A.M., melatonin concentrations were significantly lower following exposure to non-flickering light compared with dim light, whereas there was no significant difference between the dim and 100-Hz flickering light conditions. By contrast, after 1:30 A.M., the mean melatonin concentrations were significantly lower under both 100-Hz flickering and non-flickering light than under dim light.
Conclusion
Although melatonin suppression rate tended to be lower under 100-Hz flickering light than under non-flickering light at the initial 15 min of the light exposure, the present study suggests that 100-Hz flickering light may have the same impact on melatonin secretion as non-flickering light.