Melatonin Is Not the Whole Story: How Does Light Affect Alertness, Sleep, and Our Biological Clock?
Don’t Mistake One Hormone for the Entire Spectrum of Human Responses
Author | Lawrence Lin
Chairman, GLGA | Board Member, GLG | IWBI WELL Light Concept Advisor | LRS Founder & CEO
In discussions about healthy lighting, melatonin is often pushed to center stage. Some products use “percentage of melatonin suppression” to demonstrate alertness, while others use “less blue light” to demonstrate better sleep. But the human response to light extends far beyond a single hormonal curve.
Light can advance or delay the circadian clock, but it can also affect moment-to-moment alertness, subjective sleepiness, pupil responses, and sleep readiness. Melatonin is an important measure, but it is not the only one—and certainly not a universal health scorecard for everyone.
Three Effects: Don’t Conflate Them
The first is circadian phase. Light in the morning and at night can push the internal clock in different directions. The effect depends on when the light occurs, not simply on its intensity.
The second is the acute response. Stronger light at night may suppress melatonin and may also make people feel more alert in the moment. An acute change does not automatically translate into a long-term health benefit or harm.
The third is sleep and behavioral outcomes. Bedtime, sleep duration, daytime activity, caffeine intake, screen use, and social schedules all play a role. Lighting is an important variable, but it is not the only one.
Therefore, when a study shows “melatonin decreased,” we cannot directly translate that into “sleep will necessarily be worse.” Likewise, when people report “feeling more alert,” we cannot automatically conclude that “work performance will necessarily improve.”
Why Timing Matters More Than Color Temperature
The same light can have very different physiological effects depending on whether it appears in the morning, evening, or late at night. During the day, we need a sufficiently bright environment; before bedtime, we need to significantly reduce the amount of stimulation reaching the eyes. A single, fixed “healthy color temperature” throughout the day is often less effective than a well-defined timing strategy.
In the home, this difference is easy to see. If the living room remains as bright at night as it is during the day, it may still be too bright even if the color temperature is somewhat warmer. Conversely, if the home is lit only with dim, warm light during the day, it may fail to provide a sufficiently strong daytime signal. It is generally more prudent to address “when and how much” first, and optimize the spectrum afterward.
Why Do Research Findings So Often Differ?
What happens before a light exposure matters greatly. Someone who has spent the day outdoors may respond differently to the same light at night than someone who has spent the entire day in a dimly lit room. If a study does not report participants’ prior light exposure, exposure duration, measurement location, pupil response, or daily schedule, its findings can be difficult to compare with those of other studies.
This is what scientific caution is about: not denying that light affects people, but refusing to compress complex responses into a one-size-fits-all marketing claim.
What We Can Say Now / What We Still Can’t Say
What we can say: The amount, spectrum, timing, duration of light exposure, and prior light history can collectively influence circadian rhythms and acute responses.
What we still can’t say: A single measurement of melatonin, by itself, can prove that a particular lighting system will improve long-term sleep, mood, cognition, or disease risk.
Three Things You Can Do Today
- Design lighting strategies around daytime, pre-bedtime, and sleep periods, rather than simply dividing lighting into cool and warm color temperatures.
- When reviewing human-subject research, check whether it measures hormonal responses, subjective experience, cognitive performance, or actual sleep outcomes.
- When conducting post-occupancy or project follow-ups, include daily schedules, daytime outdoor light exposure, and screen use in the records.
Evidence status: Established consensus / Can be applied with caution.
Light can affect human circadian rhythms and acute responses. However, caution is still warranted when extrapolating from short-term biomarkers to long-term health outcomes.
References
- Brown et al., 2022, PLOS Biology：https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3001571
- IES TM-18-18: https://store.ies.org/product/tm-18-18-light-and-human-health-an-overview-of-the-impact-of-optical-radiation-on-visual-circadian-neuroendocrine-and-neurobehavioral-responses/
- CIE TN 015:2023: https://www.cie.co.at/publications/second-international-workshop-circadian-and-neurophysiological-photoreception
Next time, we’ll discuss a fact that engineers may not like—but must face: the same light will not produce exactly the same response in every person.
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