Light is a visible form of electromagnetic radiation, bordered in the spectrum by ultraviolet radiation at smaller wavelengths and infrared at larger wavelengths. Current lighting codes and guidelines provide illuminance recommendations for different room types, derived from usual lighting requirements for typical activities per room. These standards, created by technical groups such as Illuminating Engineering Society (IES), ensure good visual acuity in a variety of tasks to avoid eyestrain and to minimize productivity losses and headaches.

Light enters the eye and hits photoreceptors on the retina: rods, cones and intrinsically photosensitive retinal ganglion cells (ipRGCs). All of these cells absorb light and send it as information in the form of electrochemical signals to different parts of the brain. Rods facilitate peripheral vision and vision in dim lighting conditions, with peak sensitivity to green-blue light (498 nm). Cones facilitate daytime vision and color perception, and the peak sensitivity for the sensation of brightness with this system occurs at green-yellow light (555 nm).

In addition to facilitating vision, light influences the human body in non-visual ways. Humans and animals have internal clocks that synchronize physiological functions on roughly a 24-hour cycle called the circadian rhythm. The body responds to a number of zeitgebers—the external cues that align physiological functions to the solar day in this cycle. Light is the most important of these zeitgebers, keeping the body’s internal clocks synchronized in a process known as circadian photoentrainment.

The ipRGCs are critical to the circadian system, sending information to various parts of the brain to trigger reactions downstream in the body. These cells demonstrate peak sensitivity to teal-blue light (≈480 nm). Notably, the ipRGCs project information to a specific part of the brain called the suprachiasmatic nucleus to let it know the time of day based on the light received, and this main clock then acts as an oscillator to likewise synchronize clocks in peripheral tissues and organs.

Multiple physiological processes—including those relating to alertness, digestion and sleep—are regulated in part by the variance and interplay of hormones involved in this cycle. A consideration of light exposure is particularly significant considering the role this plays in sleep, and given that the Institute of Medicine reports that about 50 to 70 million U.S. adults have a chronic sleep or wakefulness disorder. Further, such disorders and chronic sleep deprivation are associated with increased risk of certain morbidities, including diabetes, obesity, depression, heart attack, hypertension and stroke.

All light—not just sunlight—can contribute to circadian photoentrainment. Given that people spend much of their waking day indoors, insufficient illumination or improper lighting design can lead to a drift of the circadian phase, especially if paired with inappropriate light exposure at night. Humans are continuously sensitive to light, and under normal circumstances, light exposure in the late night/early morning will shift our rhythms forward (phase advance), whereas exposure in the late afternoon/early night will shift our rhythms back (phase delay). To maintain optimal, properly synchronized circadian rhythms, the body requires periods of both brightness and darkness.