Sunday Light's mission
Sunday Light is founded on a simple belief: sunlight is one of the most powerful inputs into human health, and indoor life has lost it.
Our approach combines circadian neuroscience, engineering, and collaboration with leading scientists to recreate the experience of natural sunlight indoors. Light is the primary signal that sets the body clock, governing sleep, mood, energy, metabolism, and cognitive performance. Yet most people now spend around 93% of their time indoors, in light 50 to 100 times dimmer than the daylight our biology evolved under.[1]
We build the light your body actually needs: daylight-equivalent intensity, a balanced visible spectrum, and a colour temperature that shifts across the day in the way the sun does. The aim is to bring the measurable benefits of sunlight into everyday indoor life.
We track everything downstream. Sleep scores. HRV. Recovery. Strain. Readiness. We optimise nutrition, supplement stacks, exercise, and caffeine timing. Yet we rarely ask about the input that sets the clock governing all of these outputs in the first place: light.
This paper sets out the scientific foundation for our approach. It covers how light regulates the body clock, the problem with modern indoor light, and how we have engineered Sunday to address this.
The science of light and health is constantly evolving. But the direction of the evidence is clear, and it points to a conclusion that indoor lighting has never properly acted on: light is not just a utility for seeing. It is one of the most powerful and overlooked inputs into how we feel, think, sleep, and perform.
How Light Regulates the Circadian Rhythm
The circadian rhythm is the body's internal clock. It governs daily patterns in sleep and wakefulness, hormone release, body temperature, metabolism, and cognitive performance. Many factors influence this system, but light is by far its most powerful regulator.
The body runs on a daily internal cycle, marked by predictable variation in cortisol, glucose, body temperature, insulin, and melatonin. These create a temporal predisposition for resting and waking. The adaptive value of an internal clock is that it lets the body anticipate, rather than merely react to, the daily change in environment.[2]
The master clock and entrainment
All clocks depend on regular oscillation. The fundamental oscillation of the circadian cycle comes from the rhythmic production and degradation of proteins in the suprachiasmatic nucleus (SCN), a small region of the hypothalamus that acts as the brain's master clock.[2] For that biological metronome to be useful, it has to be set to the correct time of day. This setting process is called entrainment.
Light is by far the most important mechanism through which the rhythm is entrained, and continuous entrainment matters because the human clock does not naturally run to exactly 24 hours. The intrinsic human circadian period averages roughly 24.2 hours.[4] Like a watch that runs slightly fast, the cycle drifts a little later each day without correction. This is exactly what is observed in many totally blind people, who lack the light input needed to reset the clock.[2]
A separate visual pathway
Specialised light-sensitive cells in the eye send signals directly to the SCN. These are the intrinsically photosensitive retinal ganglion cells (ipRGCs), and they are distinct from the rods and cones used for conscious vision.[5] The eye, in effect, is a dual sense organ: one set of physiology for seeing, another for perceiving environmental time. The second pathway operates almost entirely outside conscious awareness. You don’t feel your circadian rhythm adjusting, however you do feel the downstream effects when it is misaligned.
ipRGCs contain a photopigment called melanopsin, with peak sensitivity around 480nm, which means these cells are most strongly activated by blue light. Light at 460nm is roughly twice as activating as 555nm green light.[6] The way ipRGCs process light is also unusual. Melanopsin is present at relatively low levels, so any single photon is less likely to be captured. This low sensitivity is offset by the ipRGCs' ability to aggregate photon exposure over several minutes, which makes them well suited to sensing the gradual change in light intensity across the day rather than the moment-to-moment detail that rods and cones detect.[5]
Living out of sync
When light exposure is consistent each day, the rhythm stays aligned with the external day. Morning light signals wakefulness and anchors the start of the day. Bright daytime light reinforces alertness, whereas as light falls in the evening, the body begins preparing for rest.
Problems arise when this signalling becomes weak or inconsistent. Spending the day in dim indoor light can blur the distinction between day and night from the body's perspective. Without a strong daytime signal, the rhythm drifts later, making it harder to feel alert during the day and harder to fall asleep at night. This mismatch, often described as living out of sync, is increasingly common in modern indoor life, and it does not require shift work or extreme behaviour to occur. Even small, repeated disruptions accumulate. Acute effects of misalignment include increased anxiety[7] and reduced cognitive and motor performance,[8] symptoms familiar to anyone who has had jet lag. There is also evidence that chronic misalignment raises the risk of metabolic and cardiovascular disease.[7]
The Problem with Indoor Light
We spend roughly 93% of our time indoors, in light that is 50 to 100 times dimmer than the daylight our biology expects.[1] Our circadian system evolved under the sun. For most of human history it received a daily signal of bright, blue-enriched light in the morning that signalled dawn and reset the clock. Today, most of us sit under 200 to 400 lux of artificial light that lacks the spectral quality to drive the photoreceptors behind circadian function.
Living out of sync with your circadian rhythm is like driving a car in the wrong gear. Everything still works, technically. But the engine labours, efficiency drops, and over time the wear shows. Sleep fragments, energy dips and mood struggles. The system is under strain, even when you cannot quite name why.
Office and home lighting standards have traditionally been designed around visibility, not circadian function.
Not All Light Is Equal: The Three Dimensions
Whether indoor light supports or disrupts circadian function depends on three dimensions working together: intensity, spectral quality, and timing. Getting one of them right is straightforward. Getting all three right in a single product, at a level that replicates what the sun does naturally, is the problem nobody had solved.
Dimension one: intensity
Brightness is measured in lux. A typical home provides around 200 to 300 lux. An overcast day outdoors provides around 5,000 lux. A sunny day delivers 50,000 to 100,000.
One important measure to also mention is not raw lux but melanopic EDI (melanopic equivalent daylight illuminance), the internationally standardised measure of how effectively a light source stimulates the body clock. It answers a specific question: how much natural daylight would produce the same biological response as this light source? Your body clock needs a minimum of around 250 melanopic EDI during the day to function well.[9] Most indoor environments fall far short, and a large body of research links that shortfall to disrupted mood, sleep, and cognitive performance.[9][10] In more severe cases, sustained lack of bright light contributes to seasonal affective disorder.[11]
A note on the metric. There is an alternative model called Circadian Stimulus (CS), developed by the Lighting Research Center, which also accounts for contributions from cone and rod photoreceptors beyond melanopsin. We use melanopic EDI throughout this paper because it is the international standard (CIE S 026:2018) and has been validated as the strongest single predictor of circadian responses in the peer-reviewed evidence.[12] Both metrics point in the same direction for daytime and evening recommendations.
The practical insight is that the light needs to be bright enough to actually matter. Dim light therapies, around 300 to 500 lux, show some benefit but not the robust effects seen at higher intensities.[9] You also do not need to spend all day in bright light: a morning exposure is often enough to anchor the rhythm for the day.
Dimension two: spectral quality
Not all light is equal biologically. The circadian system responds most strongly to blue light, specifically wavelengths around 460 to 480nm, because the ipRGCs are maximally sensitive there.[5][6] Daylight contains a high proportion of blue light, especially in the morning. Traditional incandescent and standard LED lighting contains a very different balance. That is one reason office lighting can feel lifeless compared to daylight even when the measured lux is reasonably high: the spectral composition is not the same.
Spectral quality has two distinct aspects that are easy to confuse.
The first is visual performance: how accurately a light source renders colour, measured by the Colour Rendering Index (CRI) on a scale from 0 to 100. An incandescent bulb scores 100. A standard office LED scores around 80, rendering colours noticeably less accurately than daylight.
The second is biological performance: how effectively the light stimulates the body clock, captured by melanopic EDI.
The counterintuitive part is that these two do not align. An incandescent bulb has a melanopic-to-photopic ratio of approximately 0.45.[13] It renders colour perfectly but delivers less than half the biological signal per lumen that daylight does. A low-quality cool-white LED can do the opposite: a strong biological signal, but harsh, poor colour. One is biologically weak and visually excellent; the other is biologically potent and visually inadequate.
This is also where the blue light debate needs care. In a typical CRI 80 LED, the blue emission peak at 450nm is around 2.5 to 4 times the intensity of the surrounding emission, a disproportionate concentration of blue energy relative to everything else, with deficient red and cyan content.[14] The concern is not the presence of a peak at 450nm. Sunlight contains energy at 450nm. What the research points to is that the issue is the ratio of that peak to the rest of the spectrum. The distinction that matters is not spike versus smooth. It is proportional versus disproportionate.
For circadian responses specifically, total weighted melanopic content predicts outcomes regardless of how the spectrum is distributed. Brown 2020 pooled 19 laboratory studies and found that melanopic illuminance predicts these responses across all source types on a single dose-response curve.[15] Gimenez et al. 2022 confirmed this across 29 studies and 326 data points.[16] Separately, higher-quality, more proportional spectra appear to be gentler on the eye: full-spectrum LEDs have been associated with measurably less retinal stress than commercial cold-white LEDs at the same brightness, and ocular inflammation has been found to correlate inversely with CRI.[17][18] Regulatory bodies including the CIE conclude that normal indoor LED exposure poses no retinal hazard for healthy adults,[19] but the consistent direction of the evidence is that spectral balance matters.
Dimension three: timing
The same light has different effects depending on when you receive it. Morning light advances the circadian rhythm, signalling dawn and anchoring the clock earlier in the day. Evening light delays the rhythm, pushing it later.[20] The effect is substantial: morning light supports sleep quality, mood, and alertness, while evening light containing significant blue wavelengths can disrupt sleep.[8][20]
This is where the role of blue light becomes nuanced. Blue-rich light in the morning is essential for waking you up and setting your clock. The same bright light in the evening suppresses melatonin and delays sleep. The question is not whether blue light is good or bad. It is whether your lighting can deliver it at the right time and remove it at the right time, which most lighting doesn’t allow. A fixed warm bulb protects your evening but starves your morning. A fixed cool LED supports your morning but disrupts your evening.
Sunlight solves this automatically, shifting from warm tones at sunrise through cool, blue-rich light at noon and back to warm at sunset. Your body reads this changing signal to orchestrate cortisol in the morning, alertness during the day, and melatonin onset in the evening.[1] Replicating that arc indoors requires a light that changes through the day, in the same way the sky does.
The Evidence Base for Bright Light
The research supporting bright light for circadian health, mood, alertness, and metabolism is extensive, with convergent findings across study designs. Major scientific bodies, including the American Academy of Sleep Medicine and the Sleep Research Society, recognise bright light as a legitimate, evidence-based intervention.[21] When researchers refer to bright light, they mean exposure significantly higher than typical indoor light. The common reference point is 10,000 lux delivered for 20 to 60 minutes per day, usually in the morning,[9] and the key is consistency: bright light at the same time each morning anchors the system, and everything downstream synchronises around it.
Mood and seasonal affective disorder. Seasonal affective disorder affects roughly 5% of the population in a severe form and perhaps 15% in a milder, subclinical form.[22] The leading hypothesis is that reduced winter daylight disrupts circadian rhythm and mood regulation. Major clinical trials consistently show that light therapy at 10,000 lux for around 30 minutes a day produces meaningful symptom improvement in a substantial proportion of cases.[11] The underlying insight, that light exposure patterns have a powerful influence on mental health, extends well beyond SAD to anyone living indoors.
Sleep quality. Morning bright light improves sleep quality, increases sleep duration, and advances sleep timing.[23] The mechanism is straightforward: morning light tells the circadian system that dawn has arrived, which allows sleep to consolidate at night. Evening light with significant blue content delays sleep onset, because the system reads it as a sign that it is still daytime.[8] The solution is not complex: bright, blue-rich light in the morning and afternoon, warmer and dimmer light in the evening.
Alertness and cognitive performance. Daytime bright light produces improvements in alertness and psychomotor performance comparable in magnitude to caffeine. Phipps-Nelson et al. demonstrated this in 2003, and the finding has been replicated many times since.[24] A single exposure can improve reaction time, reduce fatigue, and sharpen focus for hours. Unlike caffeine, light does not build tolerance, because it works through the circadian system itself.
Metabolic health. Light timing affects when the body preferentially burns carbohydrate versus fat, how the pancreas releases insulin, and how efficiently it processes glucose.[25] Studies of shift workers show higher rates of metabolic syndrome and diabetes when the rhythm is disrupted, while people with well-entrained rhythms tend to show better metabolic flexibility and more stable energy.
Individual variability. There is real variation in circadian sensitivity. Phillips et al. 2019 found up to a 50-fold difference between individuals in sensitivity to evening light.[26] Some people are naturally earlier or later chronotypes, and the optimal exposure for one person differs somewhat from another. But the overall pattern is consistent across the population: morning bright light anchors the rhythm and improves downstream markers. The specifics can be tuned to the individual; the principle holds.
Why Sunday Is Not a SAD Lamp
Much of the evidence above comes from research on SAD lamps, so it is worth being clear about how Sunday differs from them. The clinical research behind bright light therapy is robust, and SAD lamps offered the first causal proof that light directly shapes mood and energy. The limitation with SAD lamps isn’t the science, it is the friction of using them.
SAD lamps are point-source devices. At a realistic working distance of 40 to 60 centimetres, the actual light reaching your eye is far lower, and it drops further the moment you turn your head. To get the stated dose you have to sit still, close to the device, facing it, for the full session. That is a treatment protocol, and treatment protocols rely on discipline. Most people use them inconsistently, even when they understand the benefits.
There is also a question of how the light is meant to be used. Brown et al. 2022 recommend sustained daytime bright light throughout the day, not a single brief session in front of a panel.[9] Sunday takes that approach. Rather than a device you sit in front of for 30 minutes, it delivers daylight-equivalent light passively across a whole space, in the way being outdoors does. The light is simply there while you live your life beneath it, so the exposure is consistent and the benefits compound without any effort or scheduling.
Why Nobody Achieved Indoor Sunlight Before
Creating light that genuinely approximates sunlight is harder than it sounds, because the three dimensions actively work against each other. You need very bright light, which demands significant power and thermal management. You need spectral quality that mimics daylight, which means the right balance of wavelengths across the visible spectrum. And you need a form factor that lets people receive the right light at the right time as part of daily life, rather than as an effortful device. Solving for brightness at cool colour temperatures disrupts the evening. Solving for colour rendering with incandescent is insufficient for the body clock. Solving for stimulation with standard LEDs delivers a disproportionate spectrum. Every existing product makes a trade-off.
There is one further constraint that is easy to overlook. If a light does not look and feel beautiful in a home, none of the biology matters, because nobody will want to live with it. Most lighting systems have solved one or two of these problems. Few have solved all of them at once, affordably and at scale.
Why We Built Sunday
We founded Sunday to answer a specific question: what if the benefits of bright light did not require an effortful addition to your routine, and could instead be integrated effortlessly into everyday life as part of your indoor environment?
The evidence on bright light is strong, but medical-style devices that require conscious effort and scheduling have a persistent adoption problem.
So we built Sunday around three principles: intensity that approximates daylight, spectral quality that closely resembles daylight across the visible spectrum, and a form factor that feels like being under the sun and works passively in the background of a home. The light is simply there, at the intensity and spectral quality your circadian system needs, with no scheduling and no effort. The benefits compound because the exposure is consistent. We believe that is the architecture of a scalable solution to circadian health: not a device that depends on motivation, but an environment that supports health by default.
How Sunday Is Engineered
We designed Sunday to address all three dimensions in a single ceiling-mounted system that delivers daylight-equivalent light across a space.
Daylight-equivalent intensity. Sunday generates 34,500 lumens at full output, roughly 50 times the output of a typical SAD lamp, which enables it to deliver daylight-level intensity at a comfortable distance across a room rather than at a panel you sit in front of. The thermal load required to drive LEDs this hard is too high for conventional fixtures, so we developed a self-contained water-cooling system that sustains high output without compromising LED lifespan or spectral quality.
Daylight-quality spectrum. We use a high-CRI LED architecture engineered to closely match the CIE D65 daylight standard, the international reference for noon sunlight. The following was measured using an In.Licht Ultra spectrometer at 3,872K, a warm-white daytime working setting:
The Rg of 99.9 is a TM-30 gamut index measuring whether spectral energy at each wavelength is in correct proportion to the reference illuminant. A score this close to 100 means the blue peak at 450nm is proportional to everything else, not disproportionate as in a standard LED. In other words, our daylight is balanced, like daylight, not an artificial blue-heavy approximation. The melanopic EDI of 2,085 places Sunday at around 8.3 times the Brown et al. 2022 daytime minimum of 250.[9] At this level the circadian signal is robust regardless of where you sit in the room.
We share this reading as an illustration. The 3,872K reading represents spectral performance at one point in our colour-temperature range. CRI, R9, Rg, and related metrics all vary with colour temperature for any tunable-white product, because the LED's spectral output changes as warm and cool channels blend at different ratios, and the CRI reference illuminant itself switches from a blackbody standard to a daylight standard at 5,000K.
A circadian colour-temperature schedule. Sunday's colour temperature is tunable from 6,000K to 2,650K, following the natural arc of the sun. We provide an automated schedule synced to local sunrise and sunset: bright, blue-enriched light in the morning for entrainment, neutral midday light for sustained alertness, and warm amber light in the evening to minimise melatonin suppression. This is the only approach that addresses all three dimensions at once, delivering bright daylight-equivalent light when the body needs it and warm, dimmer, low-melanopic light in the evening.
Engineered daylight aesthetics. Our diffuser panel uses Rayleigh-scattering geometry to create a sky-like visual effect, and the result is frequently mistaken for a real skylight. That is the design intent: light that works passively in the background and requires no behavioural change.
Flicker-free light. Flicker is the rapid modulation of light output caused by the electrical signal driving the LEDs. Below around 200Hz it can cause headaches, eye strain, and fatigue even when it is not consciously visible. Wilkins et al. 1989 found that 100Hz fluorescent lighting caused double the headache incidence of high-frequency ballasts, with around 8% of workers particularly sensitive.[27] Sunday is flicker-free at higher brightness using analogue dimming (SVM 0.01, forty times below the EU mandatory limit). At lower brightness it uses PWM at 16kHz, well above the IEEE 1789 no-observable-effect threshold, and PWM can be disabled entirely in firmware for those who prefer pure analogue dimming across the full range.[28]
Emerging Research: Colour Contrast at Sunrise
One further dimension is worth noting, though the evidence is early. At sunrise the visual scene contains warm light from the sun and cool light from the overhead sky at the same time. Patterson et al. 2020 found that the retina has a dedicated circuit for detecting this spectral contrast,[29] and earlier work showed that around a quarter of neurons in the body clock are spectrally opponent.[30] This interests us because Sunday's Rayleigh-scattering panel simultaneously produces warm directional light from the reflected hotspot and cool diffuse light scattered through the panel, recreating the spatial spectral structure of outdoor daylight. We have not yet tested whether this creates an additional benefit over a single blended spectrum. It is an open avenue of research we are watching closely.
How Sunday Compares
Versus SAD lamps. Covered in its own section above: SAD lamps are effective but require sitting close to a panel for a fixed session, whereas Sunday delivers bright light passively across a space.
Versus circadian-friendly bulbs. Design-led lighting brands have done valuable work bringing warmer colour temperatures into the home. However, a replacement bulb delivers roughly 800 to 1,200 lumens, and a living room with four to six of them produces 100 to 300 lux at the eye during the day, a fraction of what the daytime consensus recommends.[9] These bulbs solve the warm evening case well, but they don’t solve for daytime intensity.
Versus red light therapy panels. Red light therapy (photobiomodulation) and circadian lighting operate on different mechanisms at different wavelengths. Red light therapy uses red and near-infrared wavelengths to target mitochondria for cellular and skin-level outcomes, over short ranges of centimetres. Sunday operates on the melanopsin pathway around 480nm, at room scale, for circadian entrainment, mood, alertness, and sleep timing. They are not substitutes. They address different problems for the same health-conscious person, and they complement each other.
Current limitations
We have not yet completed our own independent clinical trial. The scientific basis for our approach rests on the large, well-replicated, peer-reviewed literature on circadian biology and bright light summarised throughout this paper. We are investing in research to extend that evidence base.
Sunday does not emit ultraviolet radiation. It is not a substitute for outdoor sunlight in respect of vitamin D synthesis or any other UV-mediated pathway. It replicates the visible-spectrum daylight that drives circadian entrainment.
Sunday does not emit therapeutic near-infrared light. Most LEDs, including ours, lack meaningful output above 700nm. There is growing research on the effect of specific red and near-infrared wavelengths on mitochondrial function through a separate pathway,[31] and for that reason Sunday is complementary to, not a replacement for, a dedicated red-light therapy device.
Individual response to bright light varies with timing, chronotype, sleep history, and pattern of use. Phillips et al. 2019 found up to a 50-fold difference in individual sensitivity to evening light.[26] Sunday is designed to make it easy to get consistent, evidence-aligned light exposure through the day. It is not a guarantee of any specific outcome for any specific person, and it is not a medical device.
Summary
Light isn’t just a utility for seeing. It is one of the most powerful and overlooked inputs into how we feel every day: the way a bright morning lifts your mood, the alertness of midday, the wind-down into evening warmth. These are real, measurable effects that indoor lighting has never properly recreated.
The science of light and health is not settled. But it is clear that indoor lighting needs to evolve to support human health as part of our everyday routines, particularly as we spend more and more of our lives inside. That is what Sunday is for.
If you have any questions about Sunday Light, or about light and health more generally, please reach out to our team at enquiries@sundaylight.cc
References
[1] Wahl, S. et al. (2019). "The inner clock: blue light sets the human rhythm"
[2] Foster, R.G. (2014). "Sleep, circadian rhythms and health"
[3] Ouyang, Y. et al. (1998). "Resonating circadian clocks enhance fitness in cyanobacteria"
[4] Czeisler, C.A. et al. (1999). "Stability, precision, and near-24-hour period of the human circadian pacemaker"
[5] Berson, D.M., Dunn, F.A. & Takao, M. (2002). "Phototransduction by retinal ganglion cells that set the circadian clock"
[6] Lockley, S.W. et al. (2003). "High sensitivity of the human circadian melatonin rhythm to resetting by short wavelength light"
[7] Walker, W.H. et al. (2020). "Circadian rhythm disruption and mental health"
[8] Cain, S.W. et al. (2020). "Evening home lighting adversely impacts the circadian system and sleep"
[9] Brown, T.M. et al. (2022). "Recommendations for daytime, evening, and nighttime indoor light exposure"
[10] Boubekri, M. et al. (2014). "Impact of windows and daylight exposure on overall health and sleep quality of office workers"
[11] Golden, R.N. et al. (2005). "The efficacy of light therapy in the treatment of mood disorders"
[12] Brown, T.M. (2020). "Melanopic illuminance defines the magnitude of human circadian light responses"
[13] Houser, K.W. et al. (2022). "Correlated color temperature is not a suitable proxy for the biological potency of light"
[14] U.S. Department of Energy / PNNL. (2014). "True Colors: LEDs and the Relationship Between CCT, CRI, Optical Safety"
[15] Brown, T.M. (2020). "Melanopic illuminance defines the magnitude of human circadian light responses"
[16] Gimenez, M.C. et al. (2022). "Predicting melatonin suppression by light in humans"
[17] Chen, W. et al. (2023). "Effects of different spectrum of LEDs on retinal degeneration"
[18] Chen, W. et al. (2022). "Effect of LEDs with different color rendering indexes on ocular tissues"
[19] CIE. (2019). "Position Statement on the Blue Light Hazard"
[20] Blume, C. et al. (2019). "Effects of light on human circadian rhythms, sleep and mood"
[21] Moore-Ede, M.C. et al. (2023). "Lights should support circadian rhythms: evidence-based scientific consensus"
[22] Rosen, L.N. et al. (1990). "Prevalence of seasonal affective disorder at four latitudes"
[23] Crowley, S.J. et al. (2022). "Morning bright light improves sleep in adolescents with delayed sleep phase"
[24] Phipps-Nelson, J. et al. (2003). "Daytime exposure to bright light improves alertness and performance"
[25] Scheer, F.A.J.L. et al. (2009). "Adverse metabolic and cardiovascular consequences of circadian misalignment"
[26] Phillips, A.J.K. et al. (2019). "Individual differences in light-induced melatonin suppression"
[27] Wilkins, A.J. et al. (1989). "Fluorescent lighting, headaches and eye-strain"
[28] IEEE Std 1789-2015. "Recommended Practices for Modulating Current in High-Brightness LEDs"
[29] Patterson, S.S. et al. (2020). "A color vision circuit for non-image-forming vision in the primate retina"
[30] Walmsley, L. et al. (2015). "Colour as a signal for entraining the mammalian circadian clock"
[31] Shinhmar, H. et al. (2021). "Weeklong improved colour contrasts sensitivity after single 670nm exposures"
