Putting a number on wellness: Why you should care about Melanopic Equivalent Daylight Illuminance (M-EDI)

Ever wondered why you feel more energized on a sunny day or why staring at your phone late at night messes with your sleep? It’s all about how light impacts your body’s internal clock. Since the dawn of time, our bodies cleverly used the rising and setting of the sun to set our daytime and nighttime physiology. Electric lights effectively broke our connection with the sun and this has caused problems with health and wellness.  So, more and more frequently, the question “Is my light healthy?” is being asked by the public, architects, lighting designers, and regulatory agencies around the world.  

This is where Melanopic Equivalent Daylight Illuminance (m-EDI) comes in—a science term that’s actually really important for your health and well-being.  

What is m-EDI?

m-EDI measures how effective a light source is at stimulating your circadian system—the internal clock that tells your body when to wake up, stay alert, and wind down. It translates how much of the light hitting your eyes mimics the natural blue-sky daylight that has biological impact. In simpler terms, m-EDI helps us figure out whether the light around us is good for keeping us alert during the day or helping us relax at night *. 

Why does m-EDI matter? 

The fundamental and ancient connection our bodies have to light should not be ignored. Inside your eyes, there’s a special type of cell called intrinsically photosensitive retinal ganglion cells (ipRGCs). They send signals directly to your brain’s master clock to regulate timing of body functions. They’re most sensitive to blue light—like the kind you get from the sky during the day. 

The problem? Most indoor lighting has way less of this blue-sky light than natural daylight. That means if you’re spending most of your time indoors (and let’s face it, most of us do), you’re probably not getting the right kind of light to keep your body clock in sync. This can lead to sleep issues, sluggishness, and even a hit to your mood and productivity. Who wants to live in a chronic state of mild jetlag? 

Why high m-EDI during day and low at night? 

m-EDI might sound like something only scientists and architects care about, but it’s actually something that affects all of us. Clinical and field research shows that higher m-EDI exposure (>250 m-EDI) during the day keeps you alert and energized. If you’re feeling sluggish at work or home, it might be the lighting. On the dark side, lighting >10 m-EDI at night can mess with your melatonin levels, the hormone that helps you sleep. Bright lighting that has low m-EDI can help you wind down towards bedtime. At bedtime you want as little light as possible, but at least very little blue light (<1 m-EDI). 

Implementing m-EDI in Lighting Design 

Architects, designers, specifiers, and property owners are reevaluating lighting in offices, hospitals, schools, and homes to create lit spaces that are both bright and healthy. Incorporating melanopic-EDI into lighting design involves selecting light sources and designing lighting systems that provide appropriate levels of melanopic illuminance at the eye. This means considering not just the intensity of light but also its spectral quality and the direction from which it enters the eye. For instance, lighting that provides blue-sky light as with ‘circadian lighting’ like SkyView Tile can have a stronger melanopic effect, promoting alertness during the day. Such lighting adjusts its m-EDI output towards evening, reducing blue light exposure but keeping overall brightness high to keep late workers productive but not disrupt their body clocks for rest later.  

Living with m-EDI 

While scientists and designers work on integrating Mm-EDI into lighting systems, there are a few things to remember about your planned lighting environment.  

First, design with natural light. Getting early light exposure is a great way to help set your body clock. If you can catch the sunrise, so much the healthier. Natural daylight is the gold standard.  

Second, don’t rely on windows and bright lighting to yield high n-EDI light. You typically have to be within 2 meters of the window and looking out at blue sky to get real benefits. Skylights can be effective too but are very design sensitive. In addition, bright, standard LED, fluorescent or incandescent lighting has shockingly low m-EDI unless it is uncomfortably bright and glary on the worksurface.  

Finally, leverage circadian lighting experts like Bios to help you measure or optimize melanopic exposure for your environment. Their stand alone or building-integrated wellness lighting solutions for retrofit or new construction take a lot of the guesswork out of the planning. We all want equal opportunity healthy m-EDI levels in our inside spaces and Bios SkyView melanopic light can help you achieve lighting wellness equity.  

m-EDI might seem like a techy term, but it’s about something super relatable—how light impacts daily life. So next time you’re choosing or updating lighting in a workspace, think beyond brightness—think about what kind of light your body really needs. 

* The CIE introduced a metric called melanopic equivalent daylight illuminance (M-EDI) to estimate the likelihood of a light condition triggering a melanopsin-driven physiological response (with in vivo melanopsin photoreceptor peak sensitivity around 490nm [Spitschan 2019]). As described in the CIE report IES RP-46-23, M-EDI quantifies biologically relevant melanopic illuminance based on ipRGC sensitivity (measured in melanopic lux) and compares it to the melanopic illuminance of standard daylight at 6500K, D65. Measurements should be taken vertically at eye level to determine the light's biological impact.