What is circadian (human-centric) lighting?
Updated 2026-07-04
Circadian lighting, also called human-centric lighting, is light designed to support your body clock, not just to let you see. It follows the natural cycle: bright and blue-rich during the day to promote alertness and anchor your rhythm, then warm, dim and free of blue at night so it does not signal daytime. Its effect on the body clock is measured with melanopic metrics such as melanopic EDI (mEDI), defined in the standard CIE S 026, and referenced in building standards like WELL.
Vision versus the body clock
Ordinary lighting is designed for your image-forming vision: enough light, good color, comfortable glare. Circadian lighting adds a second goal, the non-image-forming system that sets your body clock, driven by special cells in the eye most sensitive to blue-cyan light around 480nm.
So circadian lighting cares not just about how bright and what color the light is, but about how much of that blue-cyan signal it delivers, and when.
How it is measured
The key quantity is how much light lands in the melanopic band that the clock's cells respond to. This is captured by melanopic EDI (melanopic equivalent daylight illuminance, or mEDI), standardized in CIE S 026 (2018). Building standards such as WELL (Feature L03, Circadian Lighting Design) set daytime melanopic targets for this reason.
In plain terms: high melanopic light by day, very low by night.
Circadian lighting versus dim-to-warm and tunable white
A tunable-white or dim-to-warm bulb slides between two fixed white LEDs, so it changes what you see, and warmer usually just means dimmer. True circadian lighting reshapes the whole spectrum, which lets it hit a target color while independently controlling the blue signal.
Because any color can be produced by many different spectra (an effect called metamerism), a well-engineered circadian light can hold a warm evening white while stripping out almost all of the blue, something a simple warm dimmer cannot do.
Frequently asked
What is circadian lighting?
Lighting designed to support your body clock as well as your vision: bright and blue-rich by day, warm and blue-free at night, following the natural daily cycle rather than holding one fixed setting.
What is melanopic light or mEDI?
Melanopic light is the portion of light that the body clock's photoreceptors respond to. It is quantified by melanopic EDI (mEDI), a standardized measure (CIE S 026) of how much a light stimulates that system, which matters far more for your rhythm than raw brightness or color.
What is human-centric lighting?
Human-centric lighting is another term for lighting designed around human biology and wellbeing, including circadian effects, visual comfort and mood, rather than only illumination.
Is dim-to-warm the same as circadian lighting?
Not quite. Dim-to-warm changes the color and brightness you see and reduces blue somewhat, but it slides between fixed LEDs. Circadian lighting reshapes the spectrum so it can control the blue signal independently of the visible color.
OIO is a circadian bulb for the home. It is a real spectral engine rather than two white LEDs, so it can hold a target color while setting the blue signal for the time of day: blue-rich by morning, engineered out by night. It publishes its spectra and is built on the melanopic framework described above.
- CIE S 026:2018. System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light.
- Brown et al. (2022). Recommendations for daytime, evening and nighttime indoor light exposure. PLOS Biology.
- Berson, Dunn & Takao (2002). Phototransduction by retinal ganglion cells that set the circadian clock. Science.
- WELL Building Standard v2, Feature L03: Circadian Lighting Design.
This guide is general information about light and circadian rhythm, not medical advice. OIO is not a medical device and does not diagnose, treat or cure any condition. If you have a persistent or serious sleep problem, talk to a clinician.