Definition:
Circadian stimulus (CS) is a metric developed by the Lighting Research Center (LRC) to estimate the acute melatonin-suppression response to light at the eye. It models how spectrum and intensity combine to influence the circadian system, returning a value from 0.1 (low effect) to 0.7+ (strong effect) under defined viewing conditions.
Why it matters:
CS gives designers and researchers a practical way to predict biological impact when planning light levels and spectra for workplaces, schools, healthcare and aged care. It complements visual metrics (e.g., lux, TM-30) by focusing on non-visual outcomes such as alertness and sleep timing.
How it works (summary):
- Takes spectral power distribution and illuminance at the eye.
- Applies a model of the circadian pathway (with weighting to short-wavelength content).
- Outputs a dimensionless CS value linked to expected melatonin suppression for a standard exposure.
Relation to melanopic EDI (CIE S 026):
- CS is an LRC modelling framework predicting melatonin suppression.
- Melanopic EDI (sometimes MEDI) is a CIE S 026 metric that standardises circadian-effective illuminance relative to daylight (D65).
- They address similar aims but are not directly interchangeable; some projects report both.
Applications:
- Daytime lighting scenes that support alertness.
- Evening scenes with lower circadian activation to protect sleep.
- Post-occupancy evaluations and field studies using spectral light dosimeters.
Related terms: