The Rhythm of the Light: A Primer in Circadian Design

Cortisol, often called the stress hormone, peaks in the morning to help us wake and become alert, then declines throughout the day. At the same time, melatonin production gradually increases. Because melatonin promotes sleep and is suppressed by light, these hormonal changes are naturally synchronized with sunrise and sunset. Bright light during the day helps reset the body’s clock, while darkness at night allows melatonin to support restful sleep.

Maintaining healthy circadian rhythms became more difficult with the introduction of electric lighting. Prior to the Industrial Revolution, people spent much of their time outdoors, allowing their biological clocks to align naturally with daylight. Today, however, lifestyles have shifted dramatically. A study by the United States Environmental Protection Agency found that people spend approximately 90 percent of their time indoors. And while this metric is specific to the United States, it is likely that similar patterns exist across other industrialized countries.

As a result, many people receive too little natural light during the day and too much artificial light at night. Without the strong signals provided by natural light cycles, the body has greater difficulty regulating hormone production.

Protecting the body’s evening wind-down process is therefore critical. Blue light exposure should be limited in the hours before bedtime — ideally beginning about two hours before sleep. This period can be considered circadian darkness, or the absence of blue light. During sleep, visual darkness, meaning complete darkness, is ideal.

When daily schedules deviate from the natural rhythm of the sun — due to work patterns or social habits — the body’s internal clock can fall out of sync. Excessive light exposure at night delays melatonin production, disrupts the body’s wind-down process, and often leads to poorer sleep and reduced performance the following day.

Researchers have linked disrupted circadian rhythms to a variety of health concerns. Psychiatrists have observed associations with mood disorders including impulsivity, mania, depression, and aggressive behavior. Sleep researchers note that many psychological disorders also involve sleep disturbances. Cancer researchers have also identified connections between circadian disruption and tumor growth, particularly in breast cancer. Other related health issues include diabetes, obesity, and heart disease.

As awareness of these issues grows, lighting designers are seeing increased interest from architects and clients in creating environments that support health and wellbeing, particularly in workplaces where people spend much of their time.

Effective circadian lighting design prioritizes daylight through windows or skylights, ideally with the brightest conditions occurring in the morning. When daylight is limited, illuminated ceilings or vertical surfaces with broad ambient light can help deliver illumination across the visual field and at the retina.

Electric lighting can also support circadian alignment through colour-tunable sources that mimic the changing colour of sunlight or fixtures designed to activate the circadian action spectrum. These systems should also prioritize visual comfort by minimizing glare, softening contrast ratios, and incorporating dimming controls.

After sundown, lighting should reduce circadian stimulation while maintaining visibility. Ambient light levels should decrease, and large illuminated surfaces should be dimmed or turned off. Well-shielded downlights, task lighting, and low-level wayfinding fixtures can provide necessary illumination while limiting light entering the direct visual field. Reducing monitor brightness and using warmer colour settings on electronic devices can also help minimize nighttime blue-light exposure.

Adapted from ​“Healthy Light”, a Schuler Shook academic presentation created by Ted Ohl, Beth Turomsha, Jim Baney, and Jess Baker.