Research articleSalivary melatonin suppression under 100-Hz flickering blue light and non-flickering blue light conditions
Introduction
Light has various kinds of physiological impact [11]. In particular, bright light during the night is known to delay the circadian phase [4,17], resulting in disruption of the circadian rhythm [6]. Bright light also suppresses melatonin secretion at night [7,15]. Melatonin is a hormone that is secreted from the pineal gland and has several physiological activities, including anticancer activity [18]. The International Agency for Research on Cancer lists shiftwork that involves circadian disruption as a probable carcinogenic factor (Group 2A).
Newly discovered photoreceptors, known as intrinsically photosensitive retinal ganglion cells (ipRGCs) or melanopsin-containing retina ganglion cells (mRGCs), project dark/bright information into the superchiasmatic nucleus, which regulates the circadian system [2,8,9,16]. ipRGCs have several properties that differ from cones and rods (traditional photoreceptors). The spectral peak sensitivity of ipRGCs is around 480 nm [2], conferring an acute light-induced melatonin suppression to blue light [3,20]. Furthermore, the depolarizing voltage response of ipRGCs grows more slowly in response to light pulse onset (long latency) and declines more slowly after the onset (sustained depolarization) compared to traditional photoreceptors [5]. Takao et al. [19] recorded electroretinograms (ERGs) of ipRGCs under different flickering light conditions and obtained steady state ERGs of ipRGCs to flickering light of 100 Hz suggesting that the sensitivity of temporal frequency is high. These findings suggest that the flickering condition of light may have an impact on our circadian system.
In our earlier study [13], we evaluated light-induced melatonin suppression under flickering and non-flickering light conditions. There was no significant difference on melatonin levels between non-flickering and 100 Hz flickering blue light. Because both the light conditions acutely suppressed melatonin secretion immediately after the light exposure, the light intensities of the earlier study might have been too bright. Present study examined the melatonin suppression under less bright condition.
Section snippets
Subjects
Fifteen male subjects aged 20–23 years (mean ± SD; 21.9 ± 1.9) gave informed consent to participate in this study. All subjects were free from medical conditions during the experimental period and had no histories of psychiatric or sleep disorders. Subjects were instructed to abstain from alcohol for a day prior to the experiment, and from caffeine and smoking for 3 h prior the experiment. For the 6 days preceding the experiment, they were asked to keep a regular sleep-wake schedule (sleep
Results
Data on three subjects was excluded from the analysis because the melatonin level was lower than the limit of detection of the RIA kit. Therefore, the data from twelve subjects are reported in this study.
The time courses of mean melatonin concentration in the three light conditions are shown in Fig. 3. Two-way, repeated measure ANOVA demonstrated a significant effect of time interval (F4.44 = 13.99; p < 0.01; ε = 0.33) and an interaction of time interval and light condition (F8.88 = 8.98;
Discussion
Mean melatonin levels under non-flickering and 100-Hz flickering light conditions were significantly lower than those under dim light conditions at 2:15 a.m, suggesting that the blue light conditions suppressed melatonin secretion. However, the rate of change in melatonin level under 100 Hz-flickering light was significantly lower than that under non-flickering light. The mean melatonin level under 100-Hz flickering light also showed a tendency to decrease compared to that of dim light at 2:00
Conclusion
Salivary melatonin levels were measured under non-flickering and 100-Hz flickering light conditions. 100-Hz flickering light acutely suppress melatonin levels compared to non-flickering light. This finding suggests that the flickering condition of light may be a new factor to consider when investigating the impact of light on our circadian system.
Contributors
TK contributed to the study design, checked the data, and translated/edited the manuscript.
YH contributed to the data analysis, conducted the experimental work, and drafted the original manuscript in Japanese.
JT contributed to the designing and the creating of the experimental lighting system. YK contributed to the study design, and checked health condition of the subjects.
TK, YH, JT, and YK have approved the final manuscript.
Role of the funding source
This work was supported by JSPS KAKENHI (Grant Number 15H04431).
CRediT authorship contribution statement
Tomoaki Kozaki: Conceptualization, Methodology, Software, Validation, Data curation, Writing - review & editing. Yuki Hidaka: Investigation, Formal analysis, Writing - original draft, Visualization. Jun-ya Takakura: Resources. Yosuke Kusano: Methodology, Supervision.
Declaration of Competing Interest
None of the authors has any potential conflict of interest including any financial, personal or other relationships that could influence their work and contribution to this study.
Acknowledgement
We thank all the healthy students for participating in the study.
References (20)
- et al.
A novel intrinsic electroretinogram response in isolated mouse retina
Neuroscience
(2017) Melatonin and the pineal gland: influence on mammalian seasonal and circadian physiology
Rev. Reprod.
(1998)- et al.
Phototransduction by retinal ganglion cells that set the circadian clock
Science
(2002) - et al.
Action spectrum for melatonin regulation in humans: evidence for a novel circadian photoreceptor
J. Neurosci.
(2001) - et al.
Bright light resets the human circadian pacemaker independent of the timing of the sleep-wake cycle
Science
(1986) - et al.
Melanopsin-expressing ganglion cells in primate retina signal colour and irradiance and project to the LGN
Nature
(2005) Disruption of circadian rhythms by light during day and night
Curr. Sleep Med. Rep.
(2017)- et al.
Melatonin rhythm is not shifted by lights that suppress nocturnal melatonin in humans under entrainment
Am. J. Physiol.
(1996) - et al.
Melanopsin-containing retinal ganglion cells: architecture, projections, and intrinsic photosensitivity
Science
(2002) - et al.
Melanopsin and rod-cone photoreceptive systems account for all major accessory visual functions in mice
Nature
(2003)
Cited by (3)
Understanding light pollution: Recent advances on its health threats and regulations
2023, Journal of Environmental Sciences (China)Citation Excerpt :The blue-enriched electric light from eReader before bedtime led to a 55.12% ± 20.12% reduction in evening plasma melatonin levels for participants, and a delay of over 1.5 hr of melatonin onset was also reported (Chang et al., 2015). Moreover, it was found that both flickering blue light and non-flickering blue light could acutely suppress salivary melatonin, and the decrease of melatonin under flickering blue light was significantly lower than the other (Kozaki et al., 2020). Public concerns have been raised due to the increased knowledge about various health outcomes of light pollution, and laws and regulations for controlling the potential implications of light pollution have been issued in some countries.