Photoentrainment in the Built Environment: Daylight/Lighting Regulations, Limitations, and Proposed New Design Strategies

The right light signal at the right time is the primary exogenous zeitgeber, entraining circadian rhythms. Daily solar light/dark cycles play a pivotal role in regulating circadian rhythms, sleep, neuroendocrine processes, cognitive performance, mood fluctuations, and physiological homeostasis. With the built environment replacing the unmediated natural environment, and the advent of electric lighting, human daily light environment has changed measurably. This raises concerns about whether current interior light conditions can support a stable and robust circadian photoentrainment. A well-entrained circadian system, vital for health, performance, and homeostasis, depends on a routine and consistent exposure to bright light during the day and almost total darkness at night. This narrative review (1) evaluates the efficacy of interior light environments for a robust photoentrainment, (2) identifies the limitations of current daylighting/lighting regulations, while (3) providing actionable design strategies for optimal photoentrainment in the built environment.