## 2 Background
### Circadian Hygiene in Healthcare Settings  
Disruption of natural light-dark cycles can lead to various physiological and behavioral changes, including sleep disturbances, fatigue, mood alterations, and impaired cognitive function [22]. The prevalence of ALAN has also influenced human lifestyle patterns, particularly with the rise of industrialization [8]. This has led to the increased necessity of night shift work in occupations such as firefighting and healthcare, where round-the-clock operations are required. These changes in work schedules further contribute to the disruption of the circadian rhythm [7; 22]. Night shift healthcare workers play a critical role in healthcare settings by providing continuous care, responding to emergencies, monitoring patients, assisting with procedures, and ensuring patient comfort and well-being throughout the night [23]. It is essential to implement measures that support the physical and mental health of these night shift staff [24]. However, designing lighting systems in healthcare spaces can be challenging due to the diverse chronobiological requirements of different individuals [25; 26]. Healthcare facilities encompass various functional areas, including patient rooms and nurse stations. Patients require dark and quiet nighttime environments to facilitate healing, whereas night shift staff require well-lit spaces to optimize their circadian synchronization with their inverted sleep/wake cycles [22; 24]. Balancing these contrasting institutional lighting needs is crucial for creating a suitable environment that promotes both patient recovery and staff well-being. Among the wide range of lighting strategies and designs, circadian lighting is designed to positively affect human circadian system by sending appropriately timed signals to the central clock in the brain [27; 28; 29]. In healthcare settings, the design of circadian lighting aims to synchronize the lighting environment with the natural circadian rhythm. This approach is intended to enhance patient outcomes and staff performance by maximizing the therapeutic and restorative properties of healthcare spaces [25; 28]. Specifically, a healthy lighting design for nurses during night shifts considers their need for alertness and cognitive functioning, while also creating a restful environment during breaks [24; 26]. Nurse-centered lighting design can be a beneficial approach in creating a healthy environment specifically tailored to the needs of night shift nurses [11; 23]. Following this approach, it is highly recommended for night shift nurses to be exposed to bright, blue-enriched light during their working hours [30]. The designer tries to provide lighting that mimics daylight with a higher color temperature (around 4000-6500 Kelvin) [30]. Additionally, the use of gradual transition lighting, starting with bright, blue-enriched light at the beginning of the shift and gradually transitioning to warmer and dimmer light towards the end of the shift, is another important consideration that can be implemented [24; 26; 30]. The gradual change in lighting can effectively signal to the body that the work shift, and thus the circadian \"day\", is coming to a close, facilitating the winding down process and preparation for sleep after work [26; 30]. Simultaneously, a patient-centered lighting design can be implemented to fulfill the chronobiological needs of patients [31]. According to this approach, dimmed lighting with lower intensity and warmer color temperatures (approximately 2700-3000 Kelvin) during nighttime hours is utilized to establish a soothing, tranquil, and sleep-promoting environment for hospitalized patients [25; 32]. In order to quantify the non-visual effects of light on circadian health, different metrics have also been recommended and are often used in studies. Table 2 summarizes these metrics and related information.