RESEARCH
Minds in Motion
Mariana G. Figueiro
LIGHTING DESIGN HAS TRADITIONALLY FOCUSED ON visual performance, safety, and aesthetics. Yet advances in our understanding of the nonvisual effects of light clearly indicate that lighting must also support a fundamental biological function: alignment of the circadian system. Misalignment of circadian rhythms is associated with impaired sleep, mood disturbances, cognitive dysfunction, and metabolic disruption. For lighting professionals, this presents a valuable opportunity to use light to improve human health, particularly for vulnerable populations such as those living with neurodegenerative diseases.
The 24-hour light-to-dark cycle is the primary zeitgeber (“timegiver”) that synchronizes the human circadian pacemaker located in the suprachiasmatic nuclei of the brain. Proper alignment of this system supports daytime alertness and facilitates restorative sleep at night. The characteristics of light that regulate circadian rhythms differ markedly from those required for visual functioning.
The human circadian system is most sensitive to shortwavelength light, peaking near 460 nanometers,1,2 and requires substantially higher light levels to shift its timing than those needed for vision. Evening and early night light exposure produces phase delays, while morning and early afternoon light produces phase advances.3 To manage this balancing act and thereby achieve better health, we need robust, bright light signals during the day to anchor our clock and minimal, dim light signals at night to facilitate the body’s transition into sleep.
To translate the science into lighting specifications, our team developed a model of human circadian phototransduction that led to the creation of the circadian stimulus (CS) metric,4,5 which quantifies the photic stimuli to the biological clock, characterizing the spectral and absolute sensitivity of the human circadian system to retinal light exposures. But, as described in the UL 28440 guideline,6 any metric can be used if you simply deliver “bright days, dim evenings, and dark nights.”
The CS scale ranges from below threshold (CS = 0 to 0.1, typical of outdoor environments at night) to a saturation point of CS = 0.7 (outdoor environments during the day) (Figure 1). Critically, the lighting that is typically found in homes and nursing homes often hovers close to a CS of 0.1. This level is often too low to effectively activate the circadian system, leading to a state of chronic circadian misalignment that can exacerbate the symptoms of neurodegenerative diseases.
Our research has focused on developing and testing a tailored lighting intervention (TLI) designed to deliver a daytime CS of at least 0.3—a level proven to be effective in clinical trials—while ensuring the evening CS remains below 0.1. This intervention is best delivered daily for at least 1 hour within 2 hours of waking, providing the strong morning signal necessary for circadian entrainment.
For lighting designers, the TLI shows that strong biological effects don’t require bright, uncomfortable light. Because the TLI targets the photoreceptors in the eye relevant to the circadian system, it achieves the needed CS at only 300 to 400 lux—far lower than traditional 10,000 lux light boxes, which are often glary and hard for patients to use.
Furthermore, the TLI uses warm color light sources, such as 3000K. Although aging reduces short-wavelength sensitivity due to lens yellowing, our model compensates by adjusting light levels accordingly, preserving CS while maintaining preferred warm light (Figure 2). But remember: what matters is light at the eye—not horizontal illuminance on the workplane.
The need for circadian-effective lighting is perhaps most acute in long-term care facilities. If you have visited a nursing home, you have likely observed that the “bright days, dim evenings, dark nights” ideal is rarely met. Instead, the lighting is typically low and constant throughout the day and night, exacerbating circadian misalignment.
Our earlier studies conducted in nursing homes demonstrated the profound impact of the TLI on individuals with Alzheimer’s disease and related dementias. The TLI significantly improved sleep quality, mood, and behavior.7 Remarkably, we showed that the impact of light on these outcomes was cumulative; the greater the duration of the TLI (i.e., up to 6 months in our research), the greater the observed improvements (Figure 3).8 These results were consistent with those from Eus van Someren and colleagues, who showed that daytime bright light exposure (>1,000 lux at the eye) consolidated rest-activity rhythms,9 improved cognition, and reduced depression over a period of 3.5 years.10
Recently, we recruited 60 participants with mild cognitive impairment (MCI) living in their own homes, rather than in controlled environments. Given the strong link between sleep and cognition, we evaluated whether our TLI would also improve cognitive performance. Using the Alzheimer’s Disease Assessment Scale-Cognitive Subscale, a standard neuropsychological assessment, we found significant cognitive improvement after 6 months of TLI, with effects becoming evident after 3 months. In another cohort of 35 individuals with MCI, exposure to 2 months of TLI improved glucose tolerance—a noteworthy finding, as elevated glucose levels are considered a potential risk factor for Alzheimer’s disease, even in nondiabetic individuals.11 This suggests that the benefits of light extend beyond the brain, impacting systemic metabolic health.
The TLI’s benefits extend beyond older adults and those with dementia, underscoring the broad importance of circadian alignment for brain health. Rescue and recovery workers from the World Trade Center remain especially vulnerable to circadian disruption and neuroinflammation due to extreme stress and environmental exposures during 9/11, and many still experience cognitive impairment and poor sleep.
Our research showed that a 2-month daily TLI intervention—1 hour within 2 hours of waking—significantly improved sleep and cognition in this group.12 We observed a clear correlation (Figure 4): higher morning exposure to circadian-effective light predicted better cognitive performance. These findings suggest that light therapy may offer a noninvasive way to support long-term cognitive and sleep health in individuals affected by severe trauma and environmental stress.
Parkinson’s disease (PD) patients frequently experience motor and nonmotor symptoms, including common sleep disturbances often linked to circadian disruption. Fatigue is another challenging symptom with limited evidence-based treatments.
We enrolled 45 PD participants who were exposed to TLI—again for 1 hour within 2 hours of waking—for 4 weeks. Actigraphy showed a significant 20-minute increase in sleep duration, and fatigue scores significantly improved by 3.4 points. It remains unclear whether reduced fatigue resulted from improved circadian alignment or better sleep, but replicating fatigue benefits previously seen in cancer patients highlights light as a powerful, versatile tool for managing the complex non-motor symptoms of PD.
The scientific community has, at times, been preoccupied with the debate over which metric is superior—CS, melanopic equivalent daylight illuminance, or Equivalent Melanopic Lux. While these discussions are vital for scientific rigor (and healthy debate is welcome), they can sometimes obscure the core message for the lighting industry and the design community. Over the past 5 years, rather than wasting our efforts discussing what metric to use, the Light and Health Research Center has focused on advancing the science and application of light and health through pragmatic, impactful studies that provide designers with the data they need to implement healthy lighting.
Our work confirms that light positively impacts human health in ways we had envisioned, and in ways we had only previously imagined. For lighting designers, the key insight is not the choice of metric to use, but the proven benefits of a validated, nonpharmacological intervention that improves sleep, mood, behavior, and cognition in vulnerable populations. The challenge now is to move past the debate and focus on implementation. “Circadian lighting” is not a niche or premium product, it is a fundamental requirement for the design of a healthy built environment. The future of lighting is not just about seeing; it is also about providing “bright days, dim evenings, and dark nights” to support human health and well-being.
THE AUTHOR
Mariana G. Figueiro, Ph.D., is Mount Sinai Endowed Professor of Light and Health Research at the Icahn School of Medicine at Mount Sinai and director of the Light and Health Research Center.
1 George C. Brainard et al. “Action Spectrum for Melatonin Regulation in Humans: Evidence for a Novel Circadian Photoreceptor.” Journal of Neuroscience, vol. 21, no. 16, 2001.
2 Khushwant Thapan, Josephine Arendt, and Debra J. Skene. “An Action Spectrum for Melatonin Suppression: Evidence for a Novel Non-Rod, Non-Cone Photoreceptor System in Humans.” Journal of Physiology, vol. 535, 2001.
3 Sat Bir S. Khalsa et al. “A Phase Response Curve to Single Bright Light Pulses in Human Subjects.” Journal of Physiology, vol. 549, 2003.
4 Mark S. Rea, Rohan Nagare, and Mariana G. Figueiro. “Modeling Circadian Phototransduction: Quantitative Predictions of Psychophysical Data.” Frontiers in Neuroscience, vol. 15, 2021.
5 Mark S. Rea, Rohan Nagare, and Mariana G. Figueiro. “Modeling Circadian Phototransduction: Retinal Neurophysiology and Neuroanatomy.” Frontiers in Neuroscience, vol. 14: 2021.
6 UL Standards and Engagement. Design Guideline for Promoting Circadian Entrainment with Light for Day-Active People, Design Guideline 24480, Edition 1. Northbrook, IL: Underwriters Laboratories, 2019.
7 Mariana G. Figueiro et al. “Tailored Lighting Intervention (TLI) for Improving Sleep-Wake Cycles in Older Adults Living with Dementia.” Frontiers in Physiology, vol. 14, 2023.
8 Mariana G. Figueiro et al. “Long-Term, All-Day Exposure to Circadian-Effective Light Improves Sleep, Mood, and Behavior in Persons with Dementia.” Journal of Alzheimer’s Disease Reports, vol. 4, no. 1, 2020.
9 Eus J. W. van Someren et al. “Indirect Bright Light Improves Circadian Rest-Activity Rhythm Disturbances in Demented Patients.” Biological Psychiatry, vol. 41, no. 9, 1997.
10 Rixt F. Riemersma-van der Lek et al. “Effect of Bright Light and Melatonin on Cognitive and Noncognitive Function in Elderly Residents of Group Care Facilities: A Randomized Controlled Trial.” Journal of the American Medical Association, vol. 299, no. 22, 2008.
11 Paul K. Crane et al. “Glucose Levels and Risk of Dementia.” New England Journal of Medicine, vol. 369, no. 15, 2013.
12 Ola A. Alsalman et al. “Tailored Light Intervention for Sleep and Cognition in World Trade Center (WTC) Cohort.” Archives of Environmental and Occupational Health, vol. 80, no. 9, 2025.