# A Study on the Impact of Blue Light and Its Brightness on Long-Term Memory
## 1 Introduction  
These days, people keep experiencing longer and more frequent exposure to light thanks to rapid advancements in lighting and digital technologies. Moreover, the identification of the direct and indirect effects of light on humanity has spurred the conduct of relevant research across various fields. According to previous studies, light influences different biological processes associated with blood pressure, heart rate variability, melatonin, and cortisol [1; 2; 3; 4; 5; 6] but also affects cognitive aspects, including alertness, attention, and memory in humans [7; 8; 9; 10; 11; 12; 13]. These effects, thus, play a critical role in the lives of modern-day people, who spend most of their time under artificial lighting sources. This is because human health--not only biological health but also cognitive capabilities--may be significantly affected by both the type of light to which people are exposed and the duration of their exposures. In this light, extensive research has been conducted to explore the main causes and mechanisms behind such effects of light on the human body. These efforts have led to the groundbreaking recent discovery of the photoreceptor system, composed of photosensitive retinal ganglion cells (ipRGGs) situated outside the retina called melanopsin, confirming, albeit partially, the non-visual effect of light in inducing brain activities and activating physiological reactions [14]. This photoreceptor system is closely related to the circadian rhythm. Specifically, the circadian rhythm represents an internal clock that servesto regulate the sleep-wake cycle [15]. The photoreceptor system is known to be most reactive to blue light or short-wave light with a wavelength of around 480 nm [16; 17]. Previous studies on blue light have reported that two-hour exposure to light may reduce melatonin release, which is closely associated with the sleep-wake cycle [18], and that participants exhibited improved working memory performance when exposed to blue light for 30 min then when exposed to amber light [19]. Not only that, but other studies also employed fMRI analysis to confirm the contribution of exposure to blue light in activating the brain's middle frontal gyrus, which was associated with working memory performance [20; 21]. These studies suggest that blue light can be superior to other types of colored light in enhancing human cognition. While many studies have reported the positive effect of blue light on cognitive performance or working memory, a few researchers argue that red light may also be equally effective in enhancing cognitive performance. For example, a 2011 study by [PERSON] et al. reported that red light was more effective than blue light in improving the cognitive functioning of participants [22]. Therefore, confirming the superiority of blue light in memory enhancement requires a systematic reevaluation of the effectiveness of both blue light and red light. That said, further proof is required to decisively conclude that blue light is an effective memory enhancer because, despite many previous studies highlighting the contribution of blue light to improved attention and working memory, their findings are just part of the required verification, especially when memory structure is considered. [PERSON] (1968) demonstrated that rather than being a single structure, memory is a multi-hierarchical system composed of sensory memory, short-term memory (working memory), and long-term memory [23]. Consequently, confirming the effectiveness of blue light in enhancing one type of memory requires determining whether it has a similar positive effect on other types of memory. However, most relevant studies have primarily focused on the contribution of blue light to enhancing attention or working memory; only a few studies have been conducted on the effect of blue light on long-term memory [17]. Furthermore, these limited number of studies on blue light's effect on long-term memory performance have yielded mixed findings. For example, [PERSON] et al.