Not all light is equal: spectral quality, timing, intensity & what matters for health
We track everything downstream. Sleep scores. HRV. Recovery. Strain. Readiness. We optimise nutrition, supplement stacks, exercise, caffeine timing. But we rarely ask what sets the clock that governs all of these outputs in the first place.
Light is the upstream input into how we feel, how we think, how we sleep, and how we perform. And it is not just how much of it we get, but rather the timing, composition and intensity that really matters for our health.
At Sunday Light, our mission is to recreate the feeling of being outside, indoors. The warmth of morning sun on your face, the energising clarity of bright midday light, the soft glow of evening. This is something we have always taken for granted outdoors, and never truly recreated inside. To truly embed this into everyday routines, we needed to recreate the look and feel of being outdoors, delivering daylight-level brightness, and have it work passively in the background of your home, rather than a device you sit in front of for 30 minutes everyday.
This is genuinely a difficult challenge. The different dimensions of light intensity, spectral quality, and timing, can actively work against each other. Achieving daylight-equivalent brightness without compromising on colour quality and aesthetic comfort requires solving all three simultaneously. And if it does not look and feel beautiful in your home, none of the biology matters, because you wouldn’t want to use it. Here I will set out why these dimensions matter, how they often conflict, and how we have approached this at Sunday.
How Light Shapes the Way We Feel
Most of us notice immediately how different we feel on a bright sunny morning versus a dark grey one. That is not imagination. Light affects mood, energy, alertness, and cognitive performance within minutes of exposure, through pathways that are distinct from conscious vision. Over longer timescales, it also sets the circadian rhythm, which governs nearly every system in your body, from sleep and mood to metabolism and immune function.[1] There is extensive evidence showing that disruption to this system has measurable consequences for health. In modern life, it is estimated that we spend 93% of our time indoors.[2] The majority of people are not getting the light their biology evolved to expect.
One of the most misunderstood aspects of this challenge is the role of blue light. Blue light has become a concern in recent years, and some of that concern is warranted, particularly around evening exposure. But the research is equally clear that blue light during the day is one of the most potent inputs available for setting your body clock and supporting daytime alertness.[3][4] Wahl et al. 2019 concluded that short-wavelength light is the strongest synchronising agent for the human body clock, while also emphasising that chronic exposure before bedtime has serious implications for sleep quality and overall rhythm.[1]
Personally, I love aesthetic lighting in the evening. Warm tones, candle-light, low and ambient. But this felt in direct conflict with what I knew about health and performance: that I actually need brighter, blue-rich lighting in the morning to support my mood, energy and sleep. You can’t have one without compromising the other, unless you have a lighting system that enables both, which most indoor homes don’t yet have.
This is what natural light solves automatically, as the sun shifts from bright, cool morning light into dimmer, warmer evening light as the day winds down. This is exactly what we need to replicate in an indoor environment, in a world where we spend over 90% of our time inside.
Not All Light Is Equal
Most of what people know about light and health comes down to two pieces of advice: get bright light in the morning, and avoid screens before bed. Both are true. But they address only two of the dimensions that determine whether your indoor light supports or disrupts your biology.
Recreating the feeling and benefit of being outside indoors requires three dimensions of light to work together simultaneously: intensity, spectral quality, and timing. Getting any 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 a problem nobody had solved. And doing all of this in a fixture that people actually want to live with, beautifully, is harder still. Here is a closer look at all three, why they conflict, and why this matters.
Light Intensity
Most indoor lighting fails at step one: intensity. Indoor lighting has traditionally been designed for aesthetics, not for human biology. A typical home provides around 200 to 300 lux (a standard unit of light intensity as perceived by the human eye). Going outside on an overcast day provides around 5,000 lux. A sunny day delivers 50,000 to 100,000.
Your body clock needs a minimum of 250 melanopic EDI during the day to function properly.[5] Melanopic EDI (melanopic equivalent daylight illuminance) is 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? A score of 250 means the light delivers the same stimulation as 250 lux of noon daylight.
A quick 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 piece 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 (Brown 2020, Schlangen and Price 2022). Both metrics point in the same direction for daytime and evening recommendations.
Most indoor environments fall far short. There is extensive research showing the impact of this, in particular the disruption of mood, sleep and cognitive performance.[5][1] In more extreme circumstances, a sustained lack of bright light leads to the commonly known condition SAD.[6]
Achieving daylight-equivalent brightness indoors is an engineering challenge in itself. The thermal load required to produce 10,000 lux at a comfortable distance is too high for conventional fixtures. This is why we developed a unique self-contained water-cooled system to deliver over 34,500 lumens of output at world-class colour quality, without compromising on lifespan or spectral quality. This is what enables Sunday to deliver what we call ‘comfortable brightness’ across a space, which means you receive physiologically meaningful levels of light passively as part of your routine.
In our companion article, How to Use Bright Light to Improve Your Life, our founder Nat explored how the intensity and timing of light shape your mood, energy, and cognitive performance. This deeper dive piece sets out practical tips to help you use light as a tool to shape your day.
Timing
Sunlight shifts from warm reddish tones at sunrise (around 2000K) through cooler, blue-rich noon (6500K) to warm again at sunset. Your body reads this changing signal and uses it to orchestrate cortisol release in the morning, alertness during the day, and melatonin onset in the evening.[1] A fixed colour temperature indoor light cannot replicate this pattern.
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 body 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 system can deliver it at the right time and remove it at the right time. Most cannot.
Spectral Quality
Intensity and timing are covered in depth in Nat’s companion article. This article goes deeper into the third dimension, and the one that is the least understood: spectral quality. What your light is actually made of at the wavelength level, and why it determines whether your indoor environment supports or disrupts your biology.
When white light passes through a prism, it separates into its component wavelengths: violet, blue, cyan, green, yellow, orange, red. Every light source produces a different mixture of these wavelengths. The graph of that mixture (called the spectral power distribution, or SPD) shows how much energy the light produces at each wavelength from about 380nm to 780nm.
Sunlight has a smooth, continuous spectrum. Energy is distributed across all visible wavelengths without sharp peaks or gaps. This is the type of light our bodies evolved under.[1]
Different systems in your body respond to different parts of the spectrum. For example, your body clock is most sensitive around 480nm (cyan-blue), driven by a special light receptor in the eye called melanopsin.[3][4] The makeup of your light determines how much energy reaches each of these systems, independently of how bright the light appears.
Two Important Types of Spectral Quality
This is a distinction I rarely see made in lighting discussions, but it is important to understanding the challenge we faced when creating indoor sunlight.
The first is visual performance: how accurately a light source renders colours. This is what CRI (Colour Rendering Index) measures, on a scale from 0 to 100. An incandescent bulb scores 100. A standard office LED scores around 80, which means it renders colours noticeably less accurately compared to being outside.
The second is biological performance: how effectively your light affects your body clock, mood and alertness. One way to measure this is melanopic EDI, which measures how much D65 daylight (the international standard for noon sunlight) would produce the same biological response as your light source.[5]
The counterintuitive part is that these two dimensions do not align. An incandescent bulb has a melanopic/photopic ratio of approximately 0.45.[7] It delivers less than half the biological signal per lumen that daylight does. It renders colours perfectly. It provides almost no signal to your body clock.
Conversely, a lower quality cool-white LED at 6500K can deliver a strong signal to your body, but it renders colours poorly and feels harsh. It is biologically potent and visually inadequate.
What Standard LEDs Get Wrong
The blue spike in standard LEDs has become a topic of significant public concern. Some of that concern is well founded, but much of it is misplaced. There is a key distinction that has emerged from ongoing research.
The Concern Is Proportion, Not Presence
In a typical CRI 80 LED, the blue emission peak at 450nm is 2.5 to 4 times the intensity of the surrounding phosphor emission. This means the LED delivers a disproportionate concentration of blue energy relative to all other wavelengths, while red and cyan content is deficient.[8]
The concern is not necessarily the presence of a peak at 450nm. Sunlight contains energy at 450nm. What research has shown so far is what actually matters is the ratio of that peak to everything else.
Fig. 2: Spectral comparison at approximately 4000K. Left: standard CRI 80 LED with disproportionate blue spike (3.5x). Right: Sunday Light CRI 98.8 with proportional blue peak (1.3x).
There does not appear to be any published study demonstrating that the shape of the spectral curve (spike versus smooth) independently affects biology. However, what does exist is substantial evidence that for body clock responses specifically, total weighted content predicts outcomes regardless of how it 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.[9] Gimenez et al. 2022 confirmed this across 29 studies and 326 data points.[10]
The research demonstrates that the distinction that matters is not spike versus smooth shape. It is proportional versus disproportionate.
High-CRI LEDs Produce Measurably Better Outcomes
The blue spike in lower quality LEDs has been shown to have negative effects. In animal studies, LEDs with disproportionate blue spikes have been associated with increased retinal oxidative stress and photoreceptor damage compared to full-spectrum LEDs at the same brightness.[11][12] While regulatory bodies including the CIE and ICNIRP conclude that normal indoor LED exposure poses no retinal hazard for healthy adults,[13] the evidence consistently shows that spectral balance matters.
Chen et al. 2023 compared full-spectrum LEDs against commercial cold-white LEDs and found that full-spectrum LEDs caused significantly less retinal stress at both 300 and 3,000 lux.[11] A companion study found ocular inflammation inversely correlated with CRI: higher colour rendering, less damage.[12] Kim et al. 2024 showed that high-CRI LEDs with warm colour temperature enhanced cellular ATP production and reduced oxidative stress, while low-CRI cool LEDs did the opposite.[14]
The pattern across these studies is consistent. Proportional spectral energy, where blue is balanced by strong content across all other wavelengths, produces measurably better biological outcomes than a spectrum dominated by a single disproportionate peak.
Why Incandescent Is Not the Answer (Even Though It Feels Like It Should Be)
Incandescent light has a perfectly smooth spectrum and CRI of 100, which makes it excellent for visual quality. The problem is that it does not address the most critical dimension for health and energy during the day: brightness.
At typical domestic brightness (50 to 150 lux), an incandescent bulb produces roughly 22 to 68 melanopic EDI. The consensus minimum for daytime biological benefit is 250.[5] In practice, this means you would need 550 to 1,100 photopic lux of incandescent light to reach the daytime threshold, which is impractical for a home.
Incandescent bulbs are excellent for the evening. Teran et al. 2026 tested 52 lamps and found incandescent produced just 1.5% melatonin suppression, versus 3.6% for warm LEDs and 12.3% for cool LEDs.[15]
However, incandescent bulbs do not meet the required level of intensity shown to support mood, alertness and body clock function during the day, and are also energy-inefficient, which makes them unviable as primary daytime lighting at scale.
A solution is needed that can address both daylight-level brightness and spectral quality.
The Right Spectrum at the Right Time
This is where the three dimensions come together, and sets out the approach we have taken with Sunday.
As mentioned above, there is significant and well-established research demonstrating that bright, blue-enriched light is the most potent signal for setting your body clock and supporting daytime mood and alertness.[16] In the evening, the opposite is true. The same consensus by Moore-Ede et al. 2023 found 82.5% agreement that evening lighting should minimise blue content.[16]
Essentially, a fixed warm bulb protects your evening but starves your morning. A fixed cool LED supports your morning but disrupts your evening.
What This Means
The summary from the research on light intensity, timing and spectrum lead to one outcome for recreating the benefits of sunlight indoors. The only approach that addresses all three dimensions is a tuneable system that schedules its spectral output across the day, delivering bright, daylight-equivalent light when your body needs it, and warm, dimmer, low-melanopic light in the evening.
Sunday Light: Our Approach
The reason something close to sunlight hasn’t been achievable before is due to the complexity of creating a solution that meets the requirements across different dimensions of light: brightness, spectral quality, and timing can often actively work against each other. Layering on top the need for it to look and feel like real sunlight in a way that works passively in a home all day poses a big challenge.
Solving for brightness at cool colour temperatures will disrupt your evening. Solving for colour rendering with incandescent is insufficient for your body clock. Solving for stimulation with standard LEDs delivers a disproportionate spectral profile. Every existing product makes a trade-off.
We built Sunday to address all three simultaneously, because we believe the way light shapes how we feel, sleep and perform is one of the most overlooked inputs in human wellbeing, and because the evidence shows that light quality is the primary environmental signal that determines how the body clock works.
The following was measured using an In.Licht Ultra spectrometer at 3872K (a warm-white daytime working setting):
Ra 98.8 | R9 96 | Rf 93.6 | Rg 99.9 | Melanopic EDI 2,085 | SVM 0.01 | Flicker Index 0.0
Fig. 4: Sunday at 2,085 melanopic EDI versus incandescent (65), typical office LED (180), and SAD lamp (~400). Brown et al. 2022 daytime minimum shown at 250.
A couple of the key numbers explained:
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. 99.9 out of 100 means the blue peak at 450nm is proportional to everything else. If it were disproportionate, this score would drop.
The melanopic EDI of 2,085 places Sunday at 8.3 times the Brown et al. minimum of 250.[5] At this level, the biological signal to your body is robust regardless of where you sit in the room.
We share this reading as an illustration. It is important to note the limitations, for example, our 3872K reading represents spectral performance at one point in our CCT range. CRI, R9, Rg, and related metrics all vary with colour temperature for any tunable-white product. The LED’s spectral output changes as warm and cool channels blend at different ratios. The CRI reference illuminant itself switches from a blackbody standard to a daylight standard at 5000K.
It is also worth noting that most LEDs, including ours, lack near-infrared output above 700nm or at meaningful levels equivalent to sunlight. There is increasing research on the impact of specific red and near-infrared wavelengths on mitochondrial function through a separate biological pathway.[17][18] This is an area we are following closely and we recommend people supplement alongside Sunday Light if they are looking for targeted infra-red therapy.
A Note on Flicker-Free Light
Another dimension to consider when it comes to health and indoor lighting is flicker. Flicker is the rapid modulation of light output caused by the electrical signal driving the LEDs. At low frequencies, typically below 200Hz, flicker can cause headaches, eye strain, and fatigue even when it is not consciously visible. Most people don’t realise their lights flicker because the effect operates below conscious perception, but the biological response is measurable.
Wilkins et al. 1989 conducted a landmark double-blind study showing that 100Hz magnetically-ballasted fluorescents caused double the headache incidence compared to 32kHz electronic ballasts. Approximately 8% of workers were particularly sensitive, averaging more than two headache days per week under the flickering lights.[19]
Sunday is flicker-free at higher brightness levels using analogue dimming (SVM 0.01, which is 40 times below the EU mandatory limit). At lower brightness, we use PWM at 16kHz, well above the IEEE 1789 no observable effect threshold. PWM can be disabled entirely via firmware for those who prefer pure analogue dimming across the full range.[20]
Emerging Research: Colour Contrast at Sunrise
One more dimension is worth noting for how light affects the body, though the evidence is early. At sunrise, the visual scene contains simultaneous warm light from the sun and cool light from the overhead sky. Patterson et al. 2020 discovered that the retina has a dedicated circuit for detecting this spectral contrast.[21] Walmsley et al. 2015 had also previously shown approximately 25% of neurons in the body clock are spectrally opponent.[22]
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 are yet to test whether this creates an additional benefit over a single blended spectrum, however it is an interesting avenue of research that we are watching closely.
Conclusion
The reason something close to sunlight hasn’t been achievable before is due to the complexity of creating a solution that meets the requirements across different dimensions of light: brightness, spectral quality, and timing can often actively work against each other.
Our aim is to bring the benefits of bright light for mood, energy and circadian health in a way that is comfortable, adjustable throughout the day, and delivers a quality of light that renders colours as if you were outside. The result is to feel as if you are under the sun, whilst also getting light that is truly supportive for your body throughout the day.
Light isn’t just a utility. It is one of the most powerful and overlooked inputs into how we feel every day. The way a bright sunny morning lifts your mood, the alertness you feel at midday, the wind-down into evening warmth. These are real, measurable effects that indoor lighting has never properly recreated. That is what Sunday is for.
The science of light and health is not settled. However, 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 time inside.
If you have any questions about Sunday Light or light and health, please reach out to our team via enquiries@sundaylight.cc
References
[1] Wahl, S. et al. (2019). “The inner clock: blue light sets the human rhythm” ↩
[2] Klepeis, N.E. et al. (2001). “The National Human Activity Pattern Survey” ↩
[3] Brainard, G.C. et al. (2001). “Action spectrum for melatonin regulation in humans” ↩
[4] Thapan, K. et al. (2001). “An action spectrum for melatonin suppression” ↩
[5] Brown, T.M. et al. (2022). “Recommendations for daytime, evening, and nighttime indoor light exposure” ↩
[6] Lam, R.W. & Levitt, A.J. (2006). “Canadian Consensus Guidelines for the Treatment of Seasonal Affective Disorder” ↩
[7] Houser, K.W. et al. (2022). “Correlated color temperature is not a suitable proxy for the biological potency of light” ↩
[8] U.S. Department of Energy / PNNL. (2014). “True Colors: LEDs and the Relationship Between CCT, CRI, Optical Safety” ↩
[9] Brown, T.M. (2020). “Melanopic illuminance defines the magnitude of human circadian light responses” ↩
[10] Gimenez, M.C. et al. (2022). “Predicting melatonin suppression by light in humans” ↩
[11] Chen, W. et al. (2023). “Effects of different spectrum of LEDs on retinal degeneration” ↩
[12] Chen, W. et al. (2022). “Effect of LEDs with different color rendering indexes on ocular tissues” ↩
[13] CIE. (2019). “Position Statement on the Blue Light Hazard” ↩
[14] Kim, J. et al. (2024). “The effects of light emitting diodes on mitochondrial function and cellular viability” ↩
[15] Teran, E. et al. (2026). “Home lighting, blue-light filtering, and their effects on melatonin suppression” ↩
[16] Moore-Ede, M.C. et al. (2023). “Lights should support circadian rhythms: evidence-based scientific consensus” ↩
[17] Barrett, E.M. & Jeffery, G. (2026). “LED lighting undermines human visual performance unless supplemented by wider spectra” ↩
[18] Shinhmar, H. et al. (2021). “Weeklong improved colour contrasts sensitivity after single 670nm exposures” ↩
[19] Wilkins, A.J. et al. (1989). “Fluorescent lighting, headaches and eye-strain” ↩
[20] IEEE Std 1789-2015. “Recommended Practices for Modulating Current in High-Brightness LEDs” ↩
[21] Patterson, S.S. et al. (2020). “A color vision circuit for non-image-forming vision in the primate retina” ↩
[22] Walmsley, L. et al. (2015). “Colour as a signal for entraining the mammalian circadian clock” ↩
