Natural Light at Work: Circadian Health, Energy and Sleep

Natural light at work is not just an office perk. It is a biological timing signal. The body is not only asking whether there is enough light to read a screen. It is asking what time it really is. When the day is spent in dim indoor rooms and the evening is filled with screens, ceiling lights and late messages, the circadian clock receives a mixed signal: too little day when the body should wake up, too much light when it should wind down.

The direct office evidence is strongest for sleep, sleepiness, vitality, alertness and some performance markers. Links with glucose, blood pressure, cortisol, melatonin and autonomic recovery are biologically plausible and supported by broader circadian science, but they should not be sold as if a window could fix metabolism. That is why, when we discuss melatonin and ageing, HRV or executive fatigue, light is not a decorative topic. It is one of the most basic and least measured environmental levers at work.

There are two common mistakes. One is to sell daylight as if it could fix everything. It cannot. The other is to treat it as an architecture or employer-branding detail. That misses the point too. The useful middle ground is this: a workplace that gives a clear daytime signal and protects evening darkness can support better sleep, steadier energy and more sustainable performance.

What circadian rhythm means, and how offices disrupt it

Circadian rhythms are biological cycles of roughly 24 hours. They do not only regulate sleep. They influence body temperature, blood pressure, appetite, glucose tolerance, hormone release, attention, tissue repair and stress responses. The central clock sits in the suprachiasmatic nucleus of the hypothalamus, but its strongest external cue comes from the retina.

This is the part many workplace guides miss: the eye contains light-sensitive cells that are not mainly there to form images. They tell the brain whether the environment is bright enough to signal daytime. These melanopsin-rich cells respond strongly to blue-cyan wavelengths. So a space can feel visually bright enough for work while still giving a weak circadian signal if the light is too dim, poorly timed, poorly directed or spectrally weak.

Modern life has inverted the natural pattern. Outdoors, even a cloudy day can deliver far more light than a standard office. At night, homes, phones, hotels and late work bring artificial light into the biological evening. The problem is not that a screen is "bad" in isolation. The problem is contrast: days that are too dim and nights that are too bright.

The metric that matters: melanopic EDI

For decades, office lighting was discussed mainly in lux. Lux is useful for vision, but it does not describe light's impact on circadian pathways very well. The International Commission on Illumination addressed this with CIE S 026:2018, a metrology system for optical radiation that influences responses mediated by intrinsically photosensitive retinal ganglion cells.

In plain language, we need to know how much "day signal" reaches the eye. The practical unit used in recent consensus work is melanopic equivalent daylight illuminance, or melanopic EDI. It is expressed in lux, but it is not the same as saying "500 lux on the desk". What matters is vertical light at eye height, in the direction of view, with a spectrum the circadian system reads as biologically meaningful.

The expert consensus by Brown, Brainard, Cajochen, Czeisler and colleagues, published in PLOS Biology in 2022, gave a clear target for healthy adults on daytime schedules: during the day, aim for at least 250 lx melanopic EDI. CIE TN 015:2023 repeats that recommendation: use daylight when available, or white electric light with good melanopic efficacy when needed.

Term	What it means	How to use it safely
Photopic lux	Light weighted for daytime visual tasks.	Useful for seeing; not enough for circadian assessment.
Melanopic EDI	Equivalent daylight illuminance for melanopsin-based signalling, following CIE S 026.	The cleanest current metric for circadian day signal.
EML	Equivalent melanopic lux, used in WELL and older literature.	Useful in standards, but not identical to melanopic EDI; do not mix thresholds casually.
CS	Circadian stimulus, developed by the Lighting Research Center.	Can guide design, but does not convert 1:1 into EDI or EML.

Timing	Practical target	Why it matters
Morning and daytime	At least 250 lx melanopic EDI at the eye.	Supports daytime signalling, alertness and circadian alignment.
Last 3 hours before sleep	10 lx melanopic EDI or less.	Reduces interference with night physiology and melatonin signalling.
Sleep	1 lx melanopic EDI or less; up to 10 lx only if needed for movement.	Protects rest, cardiovascular regulation and nighttime metabolism.

This does not turn light into a miracle treatment. It gives us a usable frame. Just as we would not call anything edible good nutrition, we should not call any visually adequate office light healthy lighting.

What office studies show

The workplace evidence is still smaller than we would like, but it is useful. The honest way to read it is to separate three layers: physiological consensus on light and circadian rhythm, field studies in offices, and design studies showing how hard it can be to apply the science without creating glare, energy burden or visual discomfort.

Source	What it studied	Useful result	Limit
Brown et al., 2022	Expert consensus on indoor light exposure.	250 lx melanopic EDI by day; 10 lx before sleep; 1 lx during sleep.	Recommendation for healthy day-active adults, not an individual medical protocol.
Boubekri et al., 2014	Offices with and without windows.	More daylight was associated with better sleep and vitality.	Small observational study.
Figueiro et al., 2017	Real circadian light exposure in office workers.	More morning circadian light was associated with better sleep and alignment.	Does not prove causality by itself.
Figueiro et al., 2019	Short circadian-lighting intervention in offices.	Lower sleepiness and higher vitality, energy and alertness.	Two days, self-reported outcomes, limited sample.
Boubekri/MacNaughton et al., 2020	Daylight and views in 30 office workers.	37 minutes more sleep by actigraphy and better cognitive simulation performance.	Short crossover study in a controlled environment.
Benedetti/Maierova et al., 2022	Optimized dynamic office lighting over five workdays.	Earlier melatonin onset and peripheral heat loss before habitual bedtime.	Healthy young adults; no long-term clinical outcomes.
PNNL/DOE, 2023	Circadian-lighting pilot design in offices.	Showed that meeting melanopic targets with electric light can raise lux, glare and energy load if poorly designed.	Design pilot interrupted by the pandemic, not a full health trial.

In 2014, Boubekri and colleagues compared workers in windowless or low-daylight environments with workers who had more daylight exposure. Workers with windows reported better sleep quality and vitality, and the actigraphy subgroup showed more light exposure, a trend toward more physical activity and longer sleep duration.

In 2017, Figueiro and colleagues measured circadian-effective light exposure in office workers using calibrated devices. Higher morning exposure was associated with shorter sleep-onset latency, better sleep quality and stronger circadian entrainment. Higher circadian-effective light over the whole day was also associated with lower depression scores and better sleep.

In 2019, another Figueiro field study tested a circadian-effective lighting intervention across four office settings. When workers received more circadian-effective light at the eye, they reported lower sleepiness and higher vitality, energy and alertness. The intervention was short, but it shows that workplaces can change light exposure without rebuilding the entire office.

Two later studies add useful pieces. In the daylight-and-views study published in 2020, 30 office workers spent one week in each of two office environments: one optimized for daylight and views, one more obstructed. In the optimized condition they slept 37 minutes longer by actigraphy and performed better on cognitive simulations. In Scientific Reports 2022, Benedetti, Maierova and colleagues tested optimized dynamic lighting against reference office lighting: melatonin onset and peripheral heat loss occurred earlier relative to habitual bedtime. That suggests better timing in people with a delayed circadian tendency, without making light a universal insomnia treatment.

Recent research outside offices points in the same direction with caution. A daily diary study published in Journal of Health Psychology in 2025 followed 103 adults for up to 70 days. Morning sunlight exposure predicted better next-night sleep quality, while total duration of sunlight exposure was generally not the main factor. The practical message is useful: the body cares not only about how much light you get, but when you get it.

The other half of the protocol: darker nights

Talking about natural light without talking about darkness is only half the story. The same system that needs a strong day signal also needs a clear night signal. If a company normalizes late work, night calls and instant response culture, it cannot fully offset that with a pleasant window at 11:00.

The evidence on night light is becoming harder to dismiss. In 2022, a laboratory study in PNAS found that one night of sleep under moderate room light at 100 lux, compared with dim light below 3 lux, increased nighttime heart rate, reduced vagal modulation and increased next-morning insulin resistance in healthy adults.

In 2025, JAMA Network Open published a UK Biobank cohort of 88,905 adults with about 13 million hours of personal light exposure data and 9.5 years of follow-up. Brighter nights were associated with higher risk of coronary artery disease, myocardial infarction, heart failure, atrial fibrillation and stroke after adjustment for established risk factors. It is observational evidence, not individual proof of causality, but it fits known mechanisms involving circadian disruption, blood pressure, glucose and autonomic regulation.

For a company, the conclusion is not to police anyone's bedroom. It is simpler: avoid making biological night part of the workday by default, and teach teams that answering messages at 23:30 has a biological cost too.

Latitude and offices: the issue is not only sunlight availability

In Spain, people often assume light is not a problem because the country has abundant sun. But the circadian clock does not live in the weather report. It lives in the retina of a specific person. If someone drives from a garage, spends eight hours indoors, eats lunch in front of a screen and leaves work around dusk, their biology may receive less useful daytime light than someone in northern Europe who walks outside twice a day.

Latitude matters, especially in winter, but timing and access matter too. Spain also has late dinners, late screens and offices with static light all day. The result can be paradoxical: plenty of daylight outdoors, too little day signal at the eye and too much light late in the evening.

The practical recommendation is not uncontrolled sun exposure. It is early, repeated and safe outdoor light: step outside in the morning, move some meetings outdoors, use terraces or courtyards when available, place workstations near windows without glare, and protect skin and eyes during high-UV hours. Circadian health does not require sunburn or midday exposure.

Vitamin D: related to sunlight, but not the same metric

Two conversations often get mixed together. Circadian signalling depends on visible light reaching the retina, especially in the morning. Vitamin D depends mainly on UVB radiation reaching the skin. A window may help you see the sky and strengthen the day signal, but glass blocks much of the UVB; sitting by a window is not a reliable vitamin D strategy.

The NIH Office of Dietary Supplements notes that groups at higher risk of inadequate vitamin D include older adults, people with limited sun exposure, people with dark skin, fat-malabsorption conditions, obesity or gastric bypass surgery. The Endocrine Society updated its guideline in 2024 and urges caution around routine screening in healthy adults: not everyone needs a blood test or high-dose supplements. For adults older than 75, pregnancy or high-risk prediabetes, the conversation changes and should be individualized.

For a company, the lesson is not "send everyone into the sun". It is more specific: make safe outdoor breaks possible, pair light with movement, avoid trapping people indoors all day and, when fatigue is persistent or there is bone pain, muscle weakness, deficiency risk or very low real sun exposure, connect people with clinical assessment. Vitamin D is measured with 25-OH vitamin D when indicated; circadian rhythm is shaped by visible light, timing and nighttime darkness.

Office fatigue: when light matters, and when to look further

Office fatigue does not always come from lack of light. It may reflect insufficient sleep, anemia, hypothyroidism, depression, sleep apnea, menopause, sedentary behavior, low energy availability, excessive meetings, alcohol, pain, medication or mental load. Light becomes a strong candidate when the pattern is typical: morning grogginess, a heavy second half of the day, caffeine as a crutch, little outdoor exposure and evenings that are too bright.

A simple two-week test: wake at a similar time, get outdoor light within the first hour of the workday, take an active mid-morning break, eat away from the screen when possible, reduce intense light and screens at the end of the day, and move the body after work. If fatigue does not shift, adding another lamp is not the answer: review sleep, labs, stress and clinical health.

Exercises and active light breaks

Timing	Exercise	Realistic dose	Goal
Before work	Easy outdoor walk.	10-20 min, phone away if possible.	Day signal, low-intensity movement and less sleep inertia.
1:1 meeting	Walking meeting.	15-30 min, short agenda.	Light, steps and less sedentary conversation.
Every 60-90 min	Active break.	3-5 min: stand, walk, neck mobility, thoracic extension, nasal breathing.	Interrupt sitting and visual fatigue.
Lunch	Short walk before or after eating.	5-10 min.	Outdoor exposure, post-meal glucose and mental reset.
2-3 days/week	Basic strength.	Chair squat, hip hinge, push, row, calf raise, core.	Muscle, posture, insulin sensitivity and chronic fatigue resilience.

The office does not need to become a gym. The useful rule is to combine light, steps and interruptions to sedentary time. For people already training hard, these breaks are not more training; they are circadian and movement hygiene.

A 30-minute audit: measure before buying technology

The practical blocker in many companies is that nobody knows whether the workplace is giving people a sufficient day signal. Before renovating, run a simple audit. It does not diagnose individual health, but it reveals dark zones, glare and poor timing.

1. Choose 6-10 points: window-adjacent desks, inner desks, meeting rooms, dining area, reception and one high-focus zone.
2. Measure vertically: sensor at 1.2 m height, facing the direction of view, not just flat on the desk.
3. Use four times: 9:00, 11:00, 15:00 and end of day if late work occurs.
4. Record context: distance from window, orientation, blinds, glare, screen reflections, electric light and actual use of the space.
5. Classify zones: good day signal, insufficient signal, good light with glare, prolonged-use dark room, too intense late-day space.
6. Act in order: move uses, open or modulate blinds, change rooms, create outdoor breaks, adjust luminaires; only then consider advanced circadian systems.

If professional equipment is available, melanopic EDI is ideal. If not, vertical lux with context is not a replacement for CIE S 026, but it is better than guessing. The goal is not number obsession; it is knowing whether the eye receives day during the day and enough darkness when rest should begin.

A company protocol: 10 actions before buying technology

Action	How to apply it	What to measure
Audit vertical light	Measure at eye level, not only on the desk, in key workstations and rooms.	Vertical lux; melanopic EDI if equipment is available.
Daylight meetings	Move one-to-ones or internal calls outdoors or near windows.	Number of daylight meetings and timing.
Morning light break	10-20 minutes outside before mid-morning when feasible.	Adherence, sleepiness and afternoon caffeine.
Place high-focus work near daylight	Use brighter zones for demanding cognitive work.	Window distance, reflections and eye strain.
Avoid all-day dark meeting rooms	Do not let meeting culture trap people in dim rooms for hours.	Accumulated hours in dark rooms.
Control glare	Use blinds, diffusers, screen orientation and outdoor views without reflections.	Visual comfort, screen contrast and complaints.
Warmer late work	Reduce intensity and cool spectrum if work continues late.	Vertical lux late in the day and actual shutdown time.
Night policy	Reduce non-urgent digital work after hours.	Night messages and recovery windows.
Brief education	Teach bright days, dark nights, vitamin D, caffeine, screens and active breaks.	Behaviour change, not attendance.
Health follow-up	Connect light with sleep, stress, exercise and biomarkers when relevant.	Sleep, HRV, blood pressure, glucose, 25-OH vitamin D if indicated and fatigue.

The WELL Standard, for example, uses vertical measurements at 1.2 m above the floor and allows daylight to contribute to circadian lighting targets. The version reviewed asks that a high proportion of workstations reach melanopic thresholds during the morning or that electric light provide a sufficient base. WELL is not a medical rulebook, but the design idea is sound: measure light where the eye is, not only where the floor plan looks good.

How Progevita would integrate this in a corporate programme

At Progevita, we would treat this as a day-night signal audit, not as a one-off talk. First we measure: real outdoor exposure, vertical light in work zones, schedules, sleep, fatigue, caffeine, alcohol, steps, late meetings and, where relevant, blood pressure, glucose, HRV trends or 25-OH vitamin D.

Then we intervene by layers: move meetings, add outdoor breaks, correct dark rooms, organise screens and digital night, include short exercises and review clinical causes if fatigue does not improve. For leadership teams, the Leadership Path can use light as one recovery and mental clarity lever. For companies, Progevita corporate programmes can turn this into a protocol with aggregate data and follow-up.

The Cofrentes setting helps because it lets people practise what they later need to bring back to the office: daylight walks, earlier meals, thermal recovery, breathwork, protected sleep and less artificial night. That is also the link with our guides to forest bathing and stress and biohacking with dose and purpose: do not stack hacks; organize biological signals.

Frequently asked questions about natural light and circadian rhythm

How long should I go outside in the morning?

There is no magic number. In practice, 10-20 minutes of outdoor morning light can be a good starting point for many adults, adjusted for season, cloud cover, skin, eyes, schedule and UV risk. The key is early and repeated exposure, not one heroic weekly session.

Does looking through a window count?

It helps less than going outside because glass, distance and orientation reduce part of the visible signal reaching the eye. For vitamin D, glass also blocks much of the UVB needed at the skin. For visual comfort and wellbeing it can help; for a strong circadian cue and for vitamin D, the strategy is not the same.

Is blue light bad?

Not by itself. During the day, short-wavelength-rich light can help signal wakefulness. At night, that same signal arrives at the wrong biological time. Timing, intensity and context matter more than the color label.

What if I work in a windowless office?

Use outdoor breaks, get light before work starts, eat outside when possible, add good melanopic daytime lighting in the morning and reduce bright cool light late in the day. If space rotation is possible, reserve brighter zones for high-demand tasks.

Can natural light help fatigue?

It can help if part of the fatigue comes from a weak day signal, irregular timing, too much screen exposure or fragile sleep. But persistent, new or intense fatigue deserves clinical review: anemia, thyroid disease, sleep apnea, depression, vitamin D deficiency, medication, pain and training load can all matter.

What does vitamin D have to do with this?

It shares the sun as a context, but not the main mechanism. For circadian rhythm, we want visible light at the retina, especially in the morning. For vitamin D, we need UVB reaching skin safely, without sunburn; if deficiency risk is present, 25-OH vitamin D and supplementation should be handled clinically.

How do I know if it is working?

Track trends for 2-4 weeks: sleep timing, awakenings, afternoon sleepiness, caffeine, HRV, resting heart rate, mood and energy. If insomnia, snoring, hypertension, anxiety or persistent fatigue are present, clinical assessment matters.

References

1. Brown TM, Brainard GC, Cajochen C, et al. Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults. PLOS Biology. 2022;20(3):e3001571. PMID: 35298459. Source.
2. CIE. CIE TN 015:2023: Second International Workshop on Circadian and Neurophysiological Photometry. Source.
3. Boubekri M, Cheung IN, Reid KJ, Wang CH, Zee PC. Impact of windows and daylight exposure on overall health and sleep quality of office workers. J Clin Sleep Med. 2014;10(6):603-611. DOI: 10.5664/jcsm.3780.
4. Figueiro MG, Steverson B, Heerwagen J, et al. The impact of daytime light exposures on sleep and mood in office workers. Sleep Health. 2017;3(3):204-215. PMID: 28526259.
5. Figueiro MG, Kalsher M, Steverson BC, Heerwagen J, Kampschroer K, Rea MS. Circadian-effective light and its impact on alertness in office workers. Lighting Research & Technology. 2019;51(2):171-183. DOI: 10.1177/1477153517750006.
6. Boubekri M, Lee J, MacNaughton P, et al. The Impact of Optimized Daylight and Views on the Sleep Duration and Cognitive Performance of Office Workers. Int J Environ Res Public Health. 2020;17(9):3219. PMID: 32384634. Source.
7. Benedetti M, Maierová L, Cajochen C, Scartezzini JL, Münch M. Optimized office lighting advances melatonin phase and peripheral heat loss prior bedtime. Scientific Reports. 2022;12:4267. DOI: 10.1038/s41598-022-07522-8. Source.
8. Safranek SF, Collier JM, Baker J, Jacobsen J, Wilkerson A. Lighting for Health and Wellness Recommendations in Offices: A Circadian Lighting Pilot Project in Chicago, IL. Pacific Northwest National Laboratory. 2023. DOI: 10.2172/1971618. Source.
9. Anderson AR, Ostermiller L, Lastrapes M, Hales L. Does sunlight exposure predict next-night sleep? A daily diary study among U.S. adults. Journal of Health Psychology. 2025;30(5):962-975. DOI: 10.1177/13591053241262643.
10. Mason IC, Grimaldi D, Reid KJ, et al. Light exposure during sleep impairs cardiometabolic function. PNAS. 2022;119(12):e2113290119. PMID: 35286195.
11. Windred DP, Burns AC, Lane JM, et al. Light Exposure at Night and Cardiovascular Disease Incidence. JAMA Network Open. 2025;8(10):e2542320. Source.
12. NIH Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals. Source.
13. Demay MB, Pittas AG, Bikle DD, et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2024. PMID: 38828931. Source.
14. International WELL Building Institute. Circadian lighting design, WELL Standard. Source.