The Science behind our work
The three numbers
If you take one thing from this page, take these. In 2022, an international group of researchers published a consensus on how much light a healthy adult needs indoors, and when.
During the day: at least 250 lux melanopic EDI, measured at eye level.
In the three hours before bed: below 10.
In the bedroom: below 1.
Three things you need to know alongside them, because without these three the numbers mislead.
The measurement point is unusual. These values apply vertically, at eye level, at roughly 1.2 metres — what your eye sees when you are seated. Almost every existing lighting calculation in buildings deals with horizontal light on the desk surface. That is a different number, and usually a more flattering one.
Melanopic EDI is not the same as lux. An ordinary light meter measures photopic light: what your eye needs in order to see. Melanopic EDI measures what your biological clock registers. Because common white artificial light is relatively weak in that respect, a room can deliver 300 photopic lux and still fall well short of 250 melanopic EDI. This is the most frequently made mistake in this field.
It is a recommendation, not a standard. It applies to healthy adults and does not replace existing regulation on visual comfort, safety or energy use. Nor does it transfer directly to children, older people or night workers.
Brown TM et al. (2022). Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults. PLOS Biology 20(3):e3001571.
The body clock exists, and we know how it works
That the body has an internal clock is not a metaphor. It is a molecular mechanism that has been mapped, and for which a Nobel Prize was awarded.
Nobel Prize in Physiology or Medicine 2017 — Jeffrey C. Hall, Michael Rosbash, Michael W. Young
Awarded for the discovery of how the circadian rhythm is driven at molecular level: a genetic feedback mechanism that sustains a cycle of roughly 24 hours in almost every cell of the body.
Berson DM, Dunn FA, Takao M (2002). Phototransduction by Retinal Ganglion Cells That Set the Circadian Clock. Science 295(5557):1070–1073.
The discovery of a third kind of light-sensitive cell in the retina, alongside the rods and cones we see with. These cells contribute nothing to the image; they report to the brain how much light there is. This is why light has to enter through the eyes, and why light on the skin does not do the job.
Natural light sets the clock differently from artificial light
Wright KP Jr. et al. (2013). Entrainment of the Human Circadian Clock to the Natural Light-Dark Cycle. Current Biology 23(16):1554–1558.
Participants spent a week with natural light only, without electric lighting. Their internal clock shifted into step with solar time. In their ordinary lives, that same clock sat later, with less light during the day and more light after sunset.
Current Biology (2017). Circadian Entrainment to the Natural Light-Dark Cycle across Seasons and the Weekend.
The follow-up study asked the practical question: how much is needed? A single weekend outdoors produced roughly 69% of the shift seen after a full week. The effect was measurable in winter as well.
What happens when the rhythm is disrupted
The two sources below are of a different kind from the rest. The first is a controlled experiment: something was changed and the consequence was measured. The second is an assessment of decades of research by an international agency.
Scheer FAJL, Hilton MF, Mantzoros CS, Shea SA (2009). Adverse metabolic and cardiovascular consequences of circadian misalignment. PNAS 106(11):4453–4458.
In the laboratory, participants' sleep and meal times were shifted twelve hours out of phase with their internal clock. The satiety hormone leptin fell, blood glucose rose despite higher insulin production, the cortisol rhythm reversed completely, and blood pressure rose. In three of the eight participants, post-meal glucose reached a pre-diabetic range. This is an experiment with a small number of participants, not a population study — but it demonstrates a causal link that observational research cannot establish.
IARC Monographs Volume 124 — Night Shift Work (2020). International Agency for Research on Cancer, World Health Organization.
Night shift work has been classified by the WHO's cancer agency as probably carcinogenic to humans (Group 2A). Worldwide, an estimated one in five workers regularly works at night.
What large-scale measurement shows
Until recently, light exposure in population research was usually asked about. For some years now it has been measured, with sensors people wear for a week. That produced the three studies below, all drawing on the same large British dataset.
Read them with one caveat in mind, one the authors make themselves: these are associations, not established causes. See the closing section of this page.
Windred DP et al. (2024). Brighter nights and darker days predict higher mortality risk: A prospective analysis of personal light exposure in >88,000 individuals. PNAS 121(43):e2405924121.
Nearly 89,000 people wore a light sensor for a week — more than 13 million hours of measurement in total — and were followed for an average of eight years. More light at night was associated with a higher risk of death; more daylight during the day with a lower risk. The association was clearest for cardiovascular and metabolic causes.
Read the study →
Burns AC et al. (2023). Day and night light exposure are associated with psychiatric disorders: an objective light study in >85,000 people. Nature Mental Health 1:853–862.
Among nearly 87,000 adults, more night-time light was associated with a raised risk of depression, anxiety disorder, post-traumatic stress disorder, psychosis and bipolar disorder. Independently of that, more daylight was associated with a lower risk.
Windred DP et al. (2024). Personal light exposure patterns and incidence of type 2 diabetes. The Lancet Regional Health – Europe 42:100943.
The same measurement method applied to type 2 diabetes, across more than 670,000 person-years of observation.
Read the study →
What this means for an ordinary morning
Science is only useful once it changes something about a Tuesday morning. Three practical conclusions, with their caveats.
Outdoors is almost always more than indoors. The differences are not small margins but orders of magnitude. The values below are common orders of magnitude from lighting engineering, not measurements of ours or figures from any single study — they vary with season, time of day, cloud cover and surroundings.
Situation | Order of magnitude (photopic lux) |
|---|---|
Clear day, outdoors | 10,000 – 100,000 |
Indoors, close to a window | around 1,000 |
Indoors, mid-room | 25 – 500 |
Overcast day, outdoors | 1,000 – 10,000 |
Grey is still daylight. An overcast December morning outdoors still delivers many times what a living room or office delivers. And because daylight is biologically more efficient than common artificial light, the difference in melanopic EDI is greater still than these photopic figures suggest. That the sun is not visible behind the cloud cover changes little.
In winter the difficulty is not the light, but the hour. In Belgium, and at comparable northern latitudes, the sun does not rise until around a quarter to nine in December. That is an hour at which most people are already indoors, at school or at work. This does not make winter a lost season, but it moves the question: from what an individual does in the morning, to how the buildings are arranged in which we spend our days.
What that means in practice for a classroom, a hospital ward or an office is exactly what we measure.
What we do not claim
The science of biological rhythms is growing quickly and remains incomplete in many places. So we are explicit about the limits of what we say.
We do not treat association as cause. The large studies on this page follow tens of thousands of people and show strong links between light and health. They do not prove that light itself causes those outcomes. Where a study is a controlled experiment, we say so.
Morning light is not a treatment. It cures nothing, and replaces no medication, therapy or medical care. Anyone with symptoms belongs with a doctor, not with us.
We do not give medical advice. We make no statements about individual health situations and do not answer personal medical questions.
Recommendations are not standards. The light values we cite are an expert consensus for healthy adults. They do not replace existing regulation on visual comfort, safety or energy use, and they do not transfer directly to children, older people or night workers.
We do not claim that artificial light achieves nothing. Light therapy is a well-researched treatment with demonstrated efficacy, including for winter depression. It is also a medical intervention, with contraindications. Anyone considering it should discuss it with a doctor.
We do not endorse products. No lamps, devices, supplements or treatments — including those of our partners. Partner contributions have no influence whatsoever on what appears on this page. We do not sell light.
We correct gladly. If you think something here is wrong, outdated or overstated, tell us at thecircadianfoundation@gmail.com.