4. Discussion
photosensitive, insomnia, sleep disorders, daytime sleepiness
In this study, we investigated the effects of morning blue LT on sleep-related parameters and mechanisms in adults with insomnia. Light has a strong, non-visual impact on behavioral and physiological functions, including sleep–wake regulation, cognitive function, and hormone secretion. These light-dependent effects are mediated mainly by intrinsically photosensitive retinal ganglion cells (ipRGCs), which contain the photopigment melanopsin. The maximum sensitivity of ipRGC is in the short-wavelength blue region of the spectrum (approximately 460–480 nm), commonly referred to as “blue light”. Classical markers of the circadian system (cortisol and clock genes) as well as subjective (self-report) and objective (actigraphy) outcomes related to sleep were studied to determine the effectiveness of LT, specifically its effects on the human circadian system using monochromatic blue-light illumination. Some improvements in subjective and objective sleep-related parameters observed in the LT group in this study are consistent with previous findings [In the present study, we were unable to confirm the impact of LT on other objective sleep parameters such as sleep efficacy, total sleep time, and wake after sleep onset, as well as subjective parameters including PSQI, ISI, and SSS. The effectiveness of LT could be influenced by several factors, including circadian rhythm, light intensity, and light duration [These elements could contribute to the negative results of a limited effectiveness of LT for insomnia in the current study. While there were no significant differences in sleep efficiency, time in bed, total sleep time, wake after sleep onset, number of awakenings, or average awakening length, it is noteworthy that we did observe a tendency for a decrease in the number of awakenings in the LT group. This observation suggests a potential positive effect of LT on reducing sleep fragmentation, which merits further exploration in larger-scale studies with longer intervention periods.Yeung et al. [We also evaluated the effect of LT by measuring serum cortisol levels. Cortisol levels in the LT group were lower than those in the control group, showing a potential trend for the effect of LT on cortisol. This is in line with previous studies showing that cortisol levels decreased after exposure to morning light [We also examined salivary melatonin levels in both groups on the first day and the last day of the clinical trial and found a tendency toward phase advancement of the melatonin circadian rhythm in the LT group in comparison to the control group. No significant changes were observed in either group. Most previous studies have focused on the effects of nighttime light exposure on non-visual-forming responses, in which light recognized by ipRGCs projects through the SCN in the brain and affects melatonin secretion in the pineal gland, which is very sensitive to short-wavelength light [To date, few studies have investigated the relationship between clock gene expression and LT. Rabia et al. [This study had several limitations. First, because we conducted this exploratory study to investigate the effect of morning LT on sleep-related parameters and the circadian system, the sample size was small. However, in this exploratory study, we observed meaningful changes in daytime sleepiness. The power was 0.531 (for ESS). Based on the current study, we are attempting to design another clinical trial with a sufficient sample size. Second, this study only involved a short trial period of two weeks, and it is still unclear whether LT has long-term efficacy in the treatment of patients with sleep disorders.Third, because light exposure is part of daily life, it is difficult to completely exclude the interference effect of artificial light. Although light is the most powerful zeitgeber, a stimulus synchronizing internal with external time, phase advancement can also occur during daytime physical exercise. Roenneberg et al. [Despite these limitations, the present study is the first to comprehensively examine the influence of morning LT on sleep-related parameters, hormones, and circadian genes.