Decoding individual differences in light sensitivity: Dr. Hou Dandan of Fudan University talks about the quantitative research and application of precise rhythmic lighting

On July 16, the "Guzhen Lighting Expo Mingren Online" and "Fudan Light" series of salons jointly invited Professor Hou Dandan, a postdoctoral fellow from the Institute of Human Phenotyping of Fudan University, to share the theme of "Analysis of Individual Difference Mechanisms of Light Regulation of Circadian Rhythm and its Application Research". The salon was hosted by Hong Bing, secretary-general of the Fudan University Alumni Association Lighting Alumni Association and editor-in-chief of China Light All Media.

Dr. Hou Dandan is deeply involved in the fields of photobiology, lighting ergonomics, and color science. He serves as a member of several technical committees of the International Commission on Illumination, participates in a number of national key R&D plans and major special projects, leads or participates in the formulation of a number of national and group rhythmic lighting standards, and holds more than ten domestic and foreign patents.

This sharing focuses on four major sections: the construction of a quantitative model of light-regulated human circadian rhythm, the establishment of a cohort based on typical light climate zones in my country, the impact of environmental and genetic factors on light-regulated circadian rhythm, and the application of precise light intervention in clinical and general lighting.

From "the third type of photoreceptor cells" to quantitative models: the non-visual effects of light are seen

Dr. Hou first reviewed the milestone discoveries in photobiology. In 2002, Professor Berson of Brown University in the United States discovered the existence of a third type of photoreceptor cells - ipRGCs - in the retina of the human eye, breaking the long-standing perception that only two types of photoreceptor cells, cones and rods, were recorded in textbooks. This discovery explains why blind people can also perceive the light and darkness of the environment and know when to sleep and when to wake up. It also opens the door to research on non-visual effects of light.

Non-imaging vision - non-visual pathway

Light not only allows people to "see" the world, but also profoundly affects the body's biological clock - regulating sleep and wakefulness, liver metabolism, heart function, and intestinal digestion capacity. This is highly consistent with the concept of "Ziwu Flow" in traditional Chinese medicine.

At the quantitative level, there are currently two types of mainstream models recognized globally: one is the CS model developed by Professor Rea's team at LRC in the United States, which uses the nighttime melatonin suppression rate as the output indicator; the other is the α-opic equivalent illumination model proposed by the Lucas team, which has been standardized and promoted by CIE. Dr. Hou pointed out that the position statement issued by CIE in 2024 emphasized: "Proper light at proper time" - the effect of light depends not only on the spectrum and intensity, but also on the time point of illumination. For the same beam of light, exposure in the morning is a benefit, while exposure at night may be a risk.

Based on this, the team of Dr. Hou and Professor Lin Yandan conducted systematic experiments focusing on the time dimension, tracking objective indicators such as core body temperature through oral capsules, and found that:

1. The light time point determines the direction and amplitude of circadian rhythm movement, playing the role of a "steering wheel"

2. Light duration is an important factor affecting the direction of phase shift, and the impact on CPS amplitude is not linearly cumulative, but an effective accumulation based on photobiological effects.

3. The level of rhythmic stimulation is an important factor affecting the amplitude. The stronger the light, the greater the intensity of promoting the rhythm

Experimental results of daily cycle photobiological effect accumulation study

The team constructed a prediction model of light intervention in human circadian rhythm phase, the DCLA effective light accumulation model, with a fitting accuracy of more than 95% on its own data, and the accuracy still reached 93% after incorporating three European external research data.

This model has been incorporated into the national standard GB/T 46119-2025 "Dose of Non-Visual Biological Effects of Light on the Human Eye" which will be officially implemented in March 2026, realizing a closed loop from basic research to industrial application.

Individual differences: a staggering 50-fold difference

Dr. Hou emphasized that behind the statistical mean that the model fits well, individual differences are a problem that cannot be ignored.

"How different are the responses of different individuals to the same light stimulus?"

Research data reveals: To achieve the same 50% suppression rate of nighttime melatonin, the most sensitive individuals only need 6 lux, while the least sensitive individuals need 350 lux, a difference of more than 50 times. Some people's circadian rhythms can shift forward or backward by 2 to 3 hours when exposed to light, while others' rhythms are almost "motionless."

"If individual differences are not addressed, healthy lighting may be effective for 50% of people and ineffective for the other 50% - about the same as tossing a coin."

Factors that affect individual light sensitivity include:

◇Age: Melatonin suppression can reach 88% in children, compared with only about 40% in adults; retinal cone cell density at age 60 is only about that at age 10 60%;

◇Myopia: Affects sleep latency and sleep duration;

◇Iris color: Light-colored iris has higher light transmittance and greater melatonin suppression rate;

◇Lighting history: People who have lived in areas with strong light for a long time are more likely to be "morning type", while people in dark-light areas are more likely to be "night type";

◇Genetic background: Multiple genetic loci such as the OPN4 gene that regulates melanin, the dopaminergic transmission pathway, and the glutamate receptor pathway

all affect light sensitivity.

Five photoclimate zone cohorts: China’s unique research advantages

In order to decipher the differences in light response among thousands of people, the team relied on China’s unique geographical advantages and based on my country’s typical light climate characteristics to build five types of photoclimate zone crowd queues. Crowd queues were established at different locations at the same latitude to explore the two core dimensions that affect human body photosensitivity.

Light environment characteristics of the zoning coverage area

Category 1 Tibet, Qinghai, northern Yunnan Strong light

Category 2 Most of Inner Mongolia and Xinjiang Stronger light

Category 3 Beijing and surrounding areas Moderate

Category 4 Northeast, Shanghai and most of the South Weak light

Category 5 Sichuan and Chongqing areas Weak light

This is a unique natural experimental field in the world, and the preliminary results are exciting:

●The first group of people: the total amount of melatonin throughout the day is higher, the circadian rhythm oscillations are larger, and the pupil contraction ability is stronger - they are more sensitive to light.

●Five categories of people: The total amount of melatonin in one day cannot be increased, and the physiological shock effect is even weaker.

● Even for people with the same genetic factors, there are significant differences in sleep duration and sleep phases under different light and climate zones - the environment has a very strong genetic modification effect.

"Back to Lhasa" is more than just a song. Research has found that Lhasa's strong natural light increases the body's serotonin levels, which in turn improves dopamine and melatonin levels, making people happy during the day and able to sleep well at night. "It turns out that there is a scientific reason for not being happy to return to Lhasa."

Precision light intervention: exploration from clinical to general lighting

In clinical applications, Dr. Hou's team has carried out a number of practices.

◆Assistant treatment for depression: In conjunction with the Shanghai Mental Health Center, patients with moderate to severe depression are treated with light-assisted intervention without discontinuing medication, and a rapid response can be seen within two weeks. The efficacy of people with high photosensitivity is significantly better than that of people with low photosensitivity, suggesting that future phototherapy should be stratified before diagnosis and treatment.

◆Rehabilitation of post-stroke cognitive impairment: At the Shanghai Third Rehabilitation Hospital, the team first classified the patients' photosensitivity and then implemented light stimulation, which significantly improved the efficiency.

◆General lighting scenario: Dr. Hou described the future of personalized light formulas - knowing the user's permanent location through mobile phone positioning, combined with age, gender, sleep time type and a simple questionnaire, the light sensitivity can be determined without the need for everyone to do genetic sequencing. By importing this information into the intelligent light control system, a personalized light formula can be output.

"From the perspective of the cost of lamps, existing smart lighting products already have the ability to adjust color temperature, brightness, CS value, EDI value, etc. What is really lacking is not the hardware, but the logic behind it - what kind of light curve should be used by people in different regions, different ages, and different light sensitivities."

Interactive Q&A

During the Q&A session, teacher Hou Dandan gave detailed answers to the audience’s questions:

Q: There are significant differences in the circadian rhythm regulation response of different individuals to light. Does this mean that future lighting products need to be customized for different groups of people? Are costs controllable?

A: From the perspective of pure lighting cost, there is not much difference from now. After years of development, smart lighting products have the ability to adjust color temperature, brightness, CS value, EDI value, etc. What really needs to change is not the product hardware, but the control logic behind it - different regions and different people should use different light curves. The cost of product functions has not been significantly improved. As for the cost of diagnosis, it is not necessary for everyone to have genetic sequencing. Through a few simple questions, combined with the location of the mobile phone to learn the light climate zone, the type of photosensitivity can be determined 90% of the time. We are developing such a questionnaire tool.

Q: What practical significance does the national standard GB/T46119-2025 "Dose of Non-Visual Biological Effects of Light on the Human Eye" have for consumers and manufacturers?

A: This standard mainly answers two questions: first, what indicators are used to quantify the non-visual effects of light. The standard combs three major indicator systems - α-opic equivalent illumination system, rhythm stimulation value CS model system and DCLA-CPS system based on the Chinese population; second, it gives the dose values that these indicators should achieve in typical application sites. Most valuable of all, the standard appendix gives correction factors for different photoclimate zones. For example, Lhasa and Chongqing have enhancement coefficients and attenuation coefficients respectively, rather than "one size fits all" across the country. China has a vast territory and great regional differences. The standards should be adapted to the life characteristics of people in different regions.

Q: What are the clinical cases of precision light intervention? For example, in the treatment of sleep disorders, rhythm regulation for shift workers, or auxiliary treatments for depression?

A: At present, attempts are mainly made in two directions: depression and cognitive rehabilitation after stroke. In terms of depression, the team worked with the Shanghai Jingwei Center to add light-assisted intervention to moderate to severe patients, which resulted in a rapid response within two weeks, and the effect was better for people with high photosensitivity. Among patients with post-stroke cognitive impairment in the Third Rehabilitation Hospital, we first performed photosensitivity classification and then performed photostimulation, and the effective rate increased by about 20% compared with previous reports. We are gradually testing this idea in different diseases, step by step.

Q: Based on a cohort study of typical light climate zones in my country, the natural light environment in different regions varies greatly. Should school lighting design adopt different rhythmic lighting schemes according to different geographical regions?

A: This is a very good question. We hope that at the practical level, whether it is home lighting, school classroom lighting or hospital lighting, we can provide differentiated standards for different regions. The first correction factors have been given in the dose standards for non-visual biological effects. For classroom lighting, in addition to the non-visual parts, there are also obvious regional differences in visual functions, color discrimination, etc. The subsequent industry will gradually improve the design specifications of partitioned and scene-based classroom rhythm lighting. We will report these results one after another.

Source: Ming classroom