Why can the same light produce different effects in different people?
Age, Sleep-Wake Patterns, Light History, and Behavior: Bringing the “Standard Person” Back to Real Populations
Author | Lawrence Lin
Chairman, GLGA | Board Member, GLG | IWBI WELL Light Concept Advisor | Founder & CEO, LRS
In senior-living projects, I have seen situations like this: all illuminance levels on the design drawings meet the requirements, and younger commissioning staff find the space bright enough, while older residents still say, “I can’t see clearly.” Elsewhere, one person feels alert by the window in the afternoon, while another remains tired because of insufficient sleep the night before.
Lighting can be standardized, but human responses can only be predicted within a certain range. Acknowledging these differences does not mean giving up on design; quite the opposite—it allows design to move from averages toward real users.
1 | Age Changes the Light That Reaches the Eyes
As people age, lens transmittance, pupil size, and visual adaptation all change. Older adults typically need more light to perform the same visual tasks and may receive less short-wavelength stimulation relevant to circadian regulation. At the same time, excessive brightness and poor glare control can cause greater discomfort.
Therefore, senior-living spaces should not simply double the overall illuminance. A more effective approach is to improve contrast, light distribution, surface reflectance, glare control, and daytime light exposure in the direction of the eyes simultaneously.
2 | Light History Changes Current Responses
The human body does not start from zero every day. How much daylight a person has received, whether the previous few hours were spent in bright or dim environments, and whether their recent sleep-wake schedule has been regular can all influence subsequent responses. Night-shift workers, people traveling across time zones, and regular daytime workers should not necessarily follow the same unadjusted schedule.
This also explains why rigorously controlled laboratory findings need to be revalidated in real buildings. People move around, pull curtains, turn their heads, work overtime, and adjust lighting themselves. Behavior itself is part of light exposure.
3 | “Human-Centric” Is Not Just a Slogan
A meaningful definition of the target population should include, at minimum, age range, primary sleep-wake schedule, activity type, time spent in the space, typical viewing directions, and specific visual needs. Schools should distinguish between children and teachers; hospitals should distinguish between patients, care staff, and night-shift doctors; residential environments should distinguish between getting up during the night, reading, and relaxing before bed.
If a project brief simply says “create a healthy and comfortable environment” without specifying who is doing what and when, the objective is almost impossible to calculate or verify.
3 | What We Can Say / What We Cannot Yet Say
What we can say: Age, pupil size, ocular media, sleep-wake patterns, previous light exposure, and behavior all contribute to differences between individuals and populations.
What we cannot yet say: That reaching a single threshold will produce the same effect for people of all ages, work schedules, and health conditions.
4 | Three Things That Can Be Done Today
- Establish a “user profile” at project kickoff, including at least age, sleep-wake patterns, and periods of occupancy.
- In spaces for older adults, prioritize visibility, contrast, and glare control before addressing circadian targets.
- For shift workers and special medical populations, involve sleep or medical professionals rather than simply copying recommendations designed for ordinary offices.
Evidence label: Existing consensus.
Human responses vary significantly; quantitative prescriptions for specific populations require more specific evidence.
References
- CIE S 026:2018:
https://www.cie.co.at/publications/cie-system-metrology-optical-radiation-iprgc-influenced-responses-light-0 - Brown et al., 2022:
https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3001571 - CIE PS 001:2024:
https://cie.co.at/publications/cie-position-statement-integrative-lighting-recommending-proper-light-proper-time-3rd
Next time, we will look at the issue from another perspective: If a human study wants others to believe its findings, how precisely does it need to describe the “light” involved?
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