The Circadian System Isn’t Just Light Sensitive. It’s Wavelength Specific

We evolved under a sky that changed not just in brightness but in color, crisp blue by day, amber dark by night, so our clocks learned to read wavelength as much as light level. Modern life flattens that signal: we spend days under dim, indoor “daylight” and evenings awash in screens and LEDs that still carry a strong blue-cyan message. Two homes can use the same number of lamps yet send opposite instructions to the brain depending on their spectral mix. Real progress comes from tuning which wavelengths reach the eye and when, not from chasing a softer glow.

Table of Contents:

1. Wavelength, not brightness

2. What I changed in practice

3. What clinicians and designers can do today

4. Measure what the brain sees

Wavelength, not brightness

The circadian system is tuned most strongly to blue–cyan wavelengths (about 460–495 nm, peaking near 480 nm) via melanopsin-containing ipRGCs, so which photons reach the eye matters more than how cozy a room looks (Moore-Ede M. et al., 2023).

Across study designs and durations, melanopic illuminance, not standard photopic lux, best predicts melatonin suppression and phase resetting, so spectrum at the eye is the variable to manage (Brown T. 2020).

Blue-enriched light in the three hours before bedtime disrupts sleep more, suppresses melatonin more, and produces larger phase delays than blue-depleted light at the same intensity, which is why timing and wavelength must be paired deliberately (Moore-Ede M. et al., 2023). Given meaningful interindividual variability, there’s strong consensus to personalize within evidence-based guardrails (Moore-Ede M. et al., 2023).

What I changed in practice

For years I said “dim and warm,” until science convinced me to change my perspective: I now plan light by melanopic dose at the eye and audit spaces the same way, rather than judging by ambience (Moore-Ede M. et al., 2023). What clinched it was seeing, in real spaces, that blue-depleted evenings can function like “virtual darkness,” advancing DLMO and nudging REM sleep without sacrificing essential visibility, exactly the kind of outcome patients need (Vethe D. et al., 2020).

I also rethought where light comes from: larger, diffuse vertical surfaces by day to raise melanopic exposure without glare, then low, indirect sources after dusk to keep light out of the eye line (Brown T. et al., 2022). I advice my patients to  programme time-of-day scenes that change light spectrum, not just dim level, blue-enriched by day, blue-depleted in the last evening hours, so the space itself “tells time” (Moore-Ede M. et al., 2023; Brown T. et al., 2022).

On the supply side, I recommend flagging high 460–495 nm LED packages as “night-risk” and keeping them out of bedrooms and corridors after dusk, aligning with experts warnings about blue-rich LEDs at night (Moore-Ede M. et al., 2023).

Hospital pilots taught me that stealth sources, hallway luminaires, nurse-station screens, backlit signage can dominate evening dose, so some studies filtered displays and relocated signage away from direct lines of sight (Vethe D. et al., 2020). 

What clinicians and designers can do today

Aim for daytime ≥250 melanopic lux, evening (last ~3 h) ≤10 melanopic lux, and sleep ≤1 melanopic lux, measured at the eye in the vertical plane, using daylight first and spectrum-aware electric light second (Brown T. et al., 2022). Pair higher-melanopic, blue-richer light with daytime tasks and swap to blue-depleted evening light to protect melatonin and circadian timing, adjusting schedules consistently day-to-day (Moore-Ede M. et al., 2023; Brown T. et al., 2022).

Choose luminaires with high melanopic DER in work zones so you can hit daytime targets without over-inflating photopic levels; in the evening, switch to low melanopic DER sources to keep tasks visible while holding biological dose down (Brown T. et al., 2022; Esposito T. & Houser K. 2022). Plan the space to support the biology: cluster morning seating near east-facing windows and use blackout/shading strategies in sleep areas to keep nighttime exposure at or below the 1-lux melanopic threshold (Brown T. et al., 2022).

In hospitals and care settings, provide night-safe orientation lighting (corridors, bathrooms) engineered to ≤10 melanopic lux at the eye; blue-depleted evening environments have been shown to preserve melatonin, advance DLMO, and modestly boost REM (Vethe D. et al., 2020; Brown T. et al., 2022). 

When setting up lighting, don’t just check how it looks. Make sure the specifications also include the light spectrum. Ask vendors for the spectrum data and melanopic values at different dimming levels, and verify these on site—so the lighting supports health as well as appearance (Esposito & Houser, 2022).

Why Expert Guidance Matters in Circadian Lighting

Designing circadian-friendly lighting is not as simple as choosing a few “warm” bulbs or dimming everything in the evening. Selecting the right products, integrating them into a space, and deciding how they should be controlled is complex. Most lighting on the market is not designed with circadian biology in mind, and two lights that seem identical can have very different melanopic effects on sleep, alertness, and health.

As a starting point, aim to maximise exposure to blue cyan wavelengths during the day, which drive higher melanopic stimulation and support alertness, and minimise exposure to these same wavelengths before sleep, using blue cyan-depleted evening lighting, whether it seems warm or cool. Correlated colour temperature (CCT) is not a reliable guide, as lights with the same apparent “warmth” can deliver very different melanopic impacts depending on their spectrum.

Finding suitable products and combining them effectively is rarely simple. An engineering consultant specialising in circadian lighting and automation can help source appropriate lights, verify their melanopic performance, and ensure they are used correctly. Advanced circadian lighting design and automation systems can make healthy light effortless, automatically adjusting spectrum and intensity throughout the day, providing brighter melanopically enriched light in the morning and evening lighting that visually suits the space but is engineered to reduce melanopic activation and protect melatonin.

For a complete implementation, a consultant designs solutions aligned with the science outlined in this article, combining spectrum, timing, and control into a seamless system. Solutions may range from fully integrated automation for new builds and renovations to simpler, non-invasive upgrades where rewiring is not an option.

The result is lighting that works with the body’s natural rhythms, not against them, supporting better sleep, sharper daytime focus, and improved wellbeing, without clients, designers, or project teams needing to manage the technical complexity.

Measure what the brain sees

Use the α-opic/CIE S 026 framework to calculate melanopic EDI from a fixture’s spectral power distribution (SPD), and make that your design target, because ipRGC-driven responses aren’t captured by appearance-based metrics (Brown T. et al., 2022).

When choosing products, ask vendors for the full light spectrum (SPDs) and include melanopic values together with standard brightness data. If spectrum data aren’t available, measure the light on site with a handheld meter or use trusted calculators based on measured spectra (Brown et al., 2022).

Don’t rely on CCT (Correlated Colour Temperature, e.g. ‘3000 K’) as a guide for biological effects. Lamps with the same CCT can have very different spectra, meaning two lights that look identical to the eye may actually send very different signals to the circadian system (Esposito & Houser, 2022).

Check where your eyes spend the most time—at desk level, in corridors, and in living areas during the evening. Remember to count light from screens (TVs, tablets) and signs, not just from lamps and ceiling fixtures (Vethe et al., 2020). 

Since people’s sensitivity to evening light depends on how much light they experienced earlier in the day, you should record their actual daytime light exposure during pilot studies. This ensures your calculations reflect real-world conditions rather than just theoretical assumptions (Brown T. et al., 2022). 

Finally, expect melanopic metrics, not photopic lux, to align best with melatonin suppression and phase shifting when you validate outcomes against your measurements (Brown T. 2020).

Conclusion

If there’s one shift that consistently moves sleep and mood in the right direction, it’s trading “how bright does it look?” for “which wavelengths hit the eye and when.” A wavelength-first mindset, designed around melanopic targets by day and strict blue-light discipline at night, turns lighting from décor into physiology. Personalize within those guardrails, measure rather than guess, and let the space “tell time” for the brain. If this resonates, share it with a colleague or patient and subscribe for monthly, practice ready protocols and case inspired insights.

References:

1. Moore-Ede M., Blask D., Cain S., Heitmann A., Nelson R. (2023). Lights Should Support Circadian Rhythms: Evidence-Based Scientific Consensus. Frontiers in Photonics. https://doi.org/10.3389/fphot.2023.1272934

2. Brown T. (2020). Melanopic illuminance defines the magnitude of human circadian light responses under a wide range of conditions. Journal of Pineal Research. https://doi.org/10.1111/jpi.12655

3. Brown T., Brainard G., Cajochen C., Czeisler C., Hanifin J., Lockley S., Lucas R., Münch M., O’Hagan J., Peirson S., Price L., Roenneberg T., Schlangen L., Skene D., Spitschan M., Vetter C., Zee P., Wright Jr K. (2022). Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults. PLOS Biology. doi: 10.1371/journal.pbio.3001571

4. Vethe D., Scott J., Engstrøm M., Salvesen Ø., Sand T., Olsen A., Morken G., Heglum H., Kjørstad K., Faaland P., Vestergaard C., Langsrud K., Kallestad H. (2020). The evening light environment in hospitals can be designed to produce less disruptive effects on the circadian system and improve sleep. Oxford Academic. Sleep. doi: 10.1093/sleep/zsaa194

5. Esposito T. & Houser K. (2022). Correlated color temperature is not a suitable proxy for the biological potency of light. Nature. https://doi.org/10.1038/s41598-022-21755-7