Circadian Metrics/SPD Data

Metrics have been developed as tools to enable lighting professionals to create environments that promote alertness by day and good sleep at night.

All circadian metric calculations require spectral power distribution (SPD) of light sources. Correlated color temperature (CCT) is not an accurate measure.

Circadian Metrics

CS characterizes the human response to light in terms of melatonin. EML and MEDI characterize a light source’s effectiveness at stimulating melanopsin. The three are not interchangeable, and each tells a different story – but any of them will indicate if one is on the right path to effective circadian lighting design, depending on the application.
Each metric provides its own calculation tool and counts toward achieving points in the WELL Building Standard, v1 or v2, in the Circadian Lighting Design category.

Image by Lighting Research Center


  • Factors in contribution of all five photoreceptors, along with amount and spectrum to assess circadian stimulation
  • Estimates the percentage of melatonin a person will suppress after one-hour exposure to a light source during the day, which in turn affects that person’s melatonin levels at night
  • Robust melatonin levels may result in better sleep, improved mood, performance, and feelings of alertness
  • High CS of >0.3 recommended for early morning, reducing to <0.1 in the evening


  • Introduces the unit ‘melanopic lux’ as a measure of light’s effect on stimulating the circadian system compared to the visual system
  • Two-part calculation involving the melanopic to photopic (M/P) ratio and illuminance at the eye (Ev)
  • The M/P ratio formula converts visual response to circadian response based on the SPD of one (or more) light sources
  • Indicates whether light source A is better or worse than light source B, of equal energy, at stimulating melanopsin
  • EML = M/P ratio x Ev


  • Factors in contribution of all five photo-receptors to determine how the ipRGCs respond to light as compared to rods and cones
  • Like EML, it is a two-part calculation requiring the melanopic daylight efficacy ratio (m-DER) and illuminance at the eye (Ev)
  • M-DER compares a light source’s ability to stimulate melanopsin to that of standard daylight:
  • MEDI = m-DER x Ev

SPD Data

Spectral Power Distributions (SPDs) of light sources are an important element of circadian lighting design. While correlated color temperature (CCT) is familiar and most often referred to as an indicator of cool or warm color appearance, it is not an accurate predictor of spectral content. Several different SPD combinations can result in the same CCT. Since the circadian system is most sensitive to short wavelength blue light in the 460 nm – 480 nm range, SPDs containing greater short-wavelength content can produce higher circadian stimulus effect.