## 2 Background  
### Circadian Clocks and Rhythms  
A circadian cycle is a series of processes occurring approximately every 24 h, which is derived from the Latin words \"circa\" and \"diem\" meaning about and day, respectively [2]. The circadian system is regulated by a central biological clock in the hypothalamic suprachi-asmatic nucleus (SCN). Different physiological and behavioral processes are coordinated through it on a 24 h basis cycle [2; 8]. Environmental cues, primarily light, stimulate the circadian clock. It is through the intrinsically photosensitive retinal ganglion cells (ipRGGs) located in the eye that light information is transmitted to the central clock in the SCN. These cells contribute specifically to the regulation of the circadian system as well as non-image-forming functions [9; 10]. ipRGGs regulate circadian rhythms by releasing the photopigment called melanopsin [11; 12]. Their photoreceptors are specifically sensitive to short-wavelength (blue) light, which makes them distinct from rods and cones that mediate visual perception [12; 13]. ipRGGs are essential for synchronizing the circadian rhythm to the external cycles of day and night, which is vital for maintaining a healthy sleep-wake patterns and circadian balance [9; 12; 13] (Figure 1).  
The widespread use of artificial lighting over the past century has introduced a new phenomenon, known as artificial light at night (ALAN), to human beings, which was absent throughout our evolutionary history [1; 8]. During the early 20 th century, when artificial electric lighting became widespread, the natural balance between light and darkness rapidly deteriorated. The transition from incandescent lighting to fluorescent lighting marked a significant milestone, offering improved efficiency and longer-lasting illumination compared to traditional options [16; 17]. Subsequently, the development of LED lighting further revolutionized the industry by providing even greater energy efficiency, durability, and design flexibility. These lighting innovations have empowered architects and designers to explore new frontiers in shaping and enhancing architectural spaces, opening up a realm of possibilities for creative and dynamic lighting designs. The advancements in lighting technology have not only improved energy efficiency but have also contributed to the aesthetic and functional aspects of architectural environments [18; 19]. However, these modern lighting advances have led to an increased uncoupling of internal circadian rhythms from the external environment driven mainly by ALAN exposure. Indeed, a significant majority (>80%) of humans no longer have the opportunity to experience naturally dark nights due to the constant presence of ALAN [20]. This disruption of the light-dark cycle has profound implications for our biological rhythms and represents a significant departure from the natural light environment under which humans have evolved for millennia [20]. The loss of dark nights is expected to increase further in the coming years. Globally, exposure to ALAN is expanding at a rate of approximately 2% annually, driven by factors such as global population growth, urbanization, and industrialization [21].  
Figure 1: Schematic diagram of the retina photoreceptors: the retina at the back of the eye with a variety of cells, including ipRGGs (intrinsically photosensitive retinal ganglion cells), adapted from [14; 15].