Light and circadian rhythm: beyond seeing
Besides cones and rods, the eye has a third receptor that is not used for vision: it sets the biological clock. Designing without it is designing only half the project.
- Reference standard
- CIE S 026 (α-opic, m-EDI) · WELL v2
- Scale
- m-EDI ≥ 250 lx (día) · ≤ 10 lx (noche)
- Where it applies
- Oficina, sanitario, residencias, hotel
- How it is measured
- Iluminancia vertical en el ojo, no en el plano de trabajo
In 2002 the intrinsically photosensitive retinal ganglion cells (ipRGC) were identified. They contain melanopsin and are most sensitive to blue light around 480 nm. They form no image: they tell the brain whether it is day or night, regulating melatonin, alertness and sleep.
The CIE S 026 standard defines how to measure this with α-opic quantities, and the practical indicator is m-EDI (melanopic Equivalent Daylight Illuminance). Current recommendations call for ≥ 250 lx m-EDI during the day at the eye, and ≤ 10 lx m-EDI in the hours before sleep. WELL v2 adopts it as a certification criterion.
The technical nuance almost nobody explains: m-EDI is measured as vertical illuminance at eye height, not as the 500 horizontal lux on the working plane. That is why an office can comfortably meet EN 12464-1 and still be circadian-poor: light reaches the desk but not the face.
The answer is therefore not to raise the level but to light vertical surfaces: wash walls and ceiling, exploit daylight and raise CCT in the morning. And the reverse at night: in care homes, hotels and hospitals, lowering level and CCT helps sleep. Human Centric Lighting systems do exactly this via DALI or wireless control.
- Prioritise vertical light and daylight: that is what reaches the eye, and it is free.
- Morning: higher level and cooler CCT; evening: lower level and warm CCT.
- In care homes and hospitals, control night light: below 10 lx m-EDI at the eye.
- Be wary of "circadian" marketing without m-EDI data: switching 3000 K to 4000 K is not enough on its own.
