Tunable white lighting has become the default answer when a client asks for human-centric or circadian-responsive design. Specify it, programme a schedule, job done. Except it isn't.
The science tells a more complicated story. And if you're specifying office lighting that genuinely needs to shift occupants' body clocks, there are things tunable white simply cannot do.
What tunable white actually does
Tunable white systems adjust correlated colour temperature (CCT), shifting a fixture from warm white (around 2700K) to cool white (around 6500K) and back. The theory is straightforward: cool, blue-enriched light in the morning mimics the rising sky and signals alertness, while warm, amber light in the evening signals wind-down.
That's a reasonable approximation of nature. The problem is that CCT is a perceptual measure, not a biological one.
A 2022 study published in Scientific Reports (Brown et al.) found that light at a single CCT can vary in circadian stimulus by up to 23%, depending on the specific spectral composition of the source. Across the full tuning range from 2500K to 6500K, two fixtures can have identical colour temperatures but deliver dramatically different biological effects. One is doing the job. The other isn't.
The melanopsin problem: why CCT alone isn't enough
Your circadian system doesn't respond to colour temperature. It responds to a specific wavelength range, peaking at around 480nm, detected by intrinsically photosensitive retinal ganglion cells (ipRGCs) that contain a photopigment called melanopsin.
Most tunable white products increase overall blue light content as they shift towards cooler temperatures. But "more blue" isn't the same as "the right kind of blue." The melanopsin response is narrow and specific. A product that looks cooler and brighter may still fail to adequately stimulate ipRGCs if its spectral output doesn't target the melanopsin action spectrum closely enough.
This is why Equivalent Melanopic Lux (EML) is now the standard measure of circadian-effective light. EML is included in WELL v2 and in the updated ISO/CIE standard 8995-1:2025. Designing to a CCT schedule instead of an EML target is a bit like designing acoustics to a colour chart. The inputs look plausible. The biological outcome misses the point entirely.
What circadian lighting for offices actually requires
To deliver genuine circadian benefit in the workplace, a system needs to work differently from a tunable white fixture running a preset schedule.
Spectral engineering, not chromatic tuning. There's a real difference between a fixture that shifts between two broadband white spectra and one that precisely targets the melanopsin-sensitive band at 480nm. The former adjusts appearance. The latter adjusts biology.
Biologically calibrated targets. Morning and midday environments should reach EML levels consistent with alertness and circadian entrainment. Evening and late-afternoon environments should stay low enough to avoid melatonin suppression. This requires control logic grounded in biological targets, not just visual comfort thresholds.
Vertical illuminance, not just horizontal. Melanopsin response depends on light reaching the eye. Most office photometric calculations optimise for horizontal illuminance on the work plane. But the light that matters for circadian entrainment is the light entering your vertical field of vision. Fixtures designed to push light onto the ceiling may actually reduce circadian stimulus, even while meeting standard lux targets on paper.
Why the spec sheet won't tell you this
Tunable white products are marketed around visible quality: Ra, R9, CRI, CCT range. These are real measures that matter for visual comfort and colour rendering. But none of them tell you the melanopic ELR — the ratio of melanopic to luminous flux — which is the value you need to assess biological performance.
Most tunable white products were not designed with ipRGC biology in mind. They were designed to shift appearance, and the circadian claim followed later. That's a commercial reality worth being honest about when you're advising a client.
Before specifying any product as a circadian lighting solution, ask the manufacturer for the melanopic ELR at each CCT setting. If they can't provide it, the product hasn't been designed to deliver circadian outcomes.
The spec community is catching up fast
The 2025 update to ISO/CIE 8995-1 has brought melanopic metrics into the mainstream. Products built from spectral biology rather than visual appearance are now available. The gap between a tunable white system that looks good and one that genuinely shifts a building's occupants' body clocks is wider than most project briefs currently acknowledge.
Your clients are asking for circadian lighting. The question is whether they're actually going to get it.
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