250, 10, 1: Scientific Consensus, or a Universal Pass/Fail Threshold?
How to use the three recommended values correctly: start with the population, time of day, and measurement conditions
Author | Lawrence Lin
Chairman, GLGA | Board Member, GLG | IWBI WELL Light Concept Advisor | Founder & CEO, LRS
“250 during the day, 10 at night, 1 while sleeping.”
These three numbers have spread rapidly in the healthy lighting community—and are also among the easiest to distort in the process of communication. Some people put them on lighting product packaging; others treat them as mandatory thresholds for every type of building.
These recommendations come from expert consensus and provide an important reference point: during the day, the eye should receive a relatively high level of mel-EDI; exposure should be significantly reduced during the three hours before bedtime; and the sleep environment should be kept as close to darkness as practical. However, the recommendations have clearly defined target populations and measurement conditions. They are not product certification thresholds that can be applied independently of context.
What Do the Three Numbers Actually Mean?
For healthy adults aged 18 to 55 with regular daytime schedules, experts recommend that, during the day, mel-EDI at eye level should ideally reach at least 250 lx, measured on a vertical plane at approximately 1.2 meters.
During the three hours before bedtime, mel-EDI at eye level should ideally be kept at no more than 10 lx.
During sleep, mel-EDI should ideally be kept at no more than 1 lx.
If a person must get up and move around at night, then, provided safety requirements are met, exposure is recommended to remain at no more than 10 lx mel-EDI, while exposure duration should be minimized.
Note that “at least” and “no more than” point in opposite directions. It is not enough to simply remember the numbers.
The Recommendations Cannot Be Applied Directly to Every Project
Older adults, children, night-shift workers, hospital patients, and people with sleep disorders may require more specific professional judgment.
Likewise, an office should not create glare simply to achieve 250 lx mel-EDI. In a hospital, nighttime lighting should never compromise patient-care or staff safety simply to reduce a numerical value.
In addition, these recommendations concern light exposure received by people, not factory-set product specifications.
If a product claims to “achieve 250 mel-EDI” without specifying the distance, direction, spatial reflectance, and dimming/control conditions, the claim has no complete engineering meaning.
Why Do You Still See 50?
Some building certifications or older provisions use EML, or specify different thresholds for particular spaces and times of day.
EML and mel-EDI are not the same quantity and should not be used interchangeably. Certification requirements may also vary by edition, compliance pathway, and applicable exemptions.
A project should first confirm which edition of which standard or certification framework applies, and then perform the corresponding calculation. Do not combine 50, 250, 10, and 1 into a single “global standard table.”
What We Can Say Today—and What We Cannot
What we can say:
250 / 10 / 1 are light-exposure recommendations for a specific population—healthy adults with regular daily schedules—based on the current body of evidence. They can serve as an important reference for lighting design.
What we cannot say:
Meeting these values does not guarantee a health outcome, and failing to meet them does not automatically mean that the lighting is harmful.
Nor should these values be treated as mandatory requirements applicable to all populations, buildings, or regulations.
Three Things You Can Do Today
- When citing these numbers, always specify the target population, time period, eye-level position, and vertical measurement direction.
- First protect visual comfort, safety, and task requirements, then optimize mel-EDI through daylight, light distribution, and lighting controls.
- For certified projects, verify the formal definition of each metric in the applicable edition. Do not convert or mix metrics on your own.
Evidence label: Use with caution.
The recommendations have a basis in expert consensus; project outcomes still need to be evaluated in the context of the specific environment and population.
References
- 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
- ANSI/IES RP-46-25：https://store.ies.org/product/recommended-practice-supporting-the-physiological-and-behavioral-effects-of-lighting-in-interior-daytime-environments/
- WELL Circadian Lighting Design (verify the applicable edition when using this reference)：https://standard.wellcertified.com/light/circadian-lighting-design
In the next article, we will unpack another common source of confusion: standards, guidelines, certifications, and regulations—what does each one actually govern?
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