Light is the most critical environmental cue for regulating the body's circadian rhythm, which is the natural, internal process that regulates the sleep-wake cycle and repeats roughly every 24 hours. Exposure to natural light during the day and darkness at night is essential for maintaining healthy circadian rhythms, which in turn influence sleep patterns, mood, and overall health. WELL Building Standards incorporate lighting strategies that mimic natural daylight patterns to support circadian health, emphasizing the importance of light in creating environments that align with human health and well-being.