Photobiology and circadian light terms
Canonical definitions for the science behind Acieon Labs. Each term is paired with its primary source from the literature.
Last updated: June 2026
- Intrinsically photosensitive retinal ganglion celliPRGC
A class of retinal neurons that express the photopigment melanopsin and respond directly to light. They project through the retinohypothalamic tract to the suprachiasmatic nucleus and regulate non-visual responses to light, including circadian timing.
Source Hattar et al., 2002; Berson et al., 2002.
- Melanopsin
The photopigment expressed in intrinsically photosensitive retinal ganglion cells. Its intrinsic light response peaks in the short-wavelength region near 480 nanometres.
Source Provencio et al., 2000; Lucas et al., 2014.
- Suprachiasmatic nucleus
A region of the hypothalamus that functions as the master circadian clock. It receives light information from intrinsically photosensitive retinal ganglion cells through the retinohypothalamic tract.
Source Hattar et al., 2002.
- Retinohypothalamic tract
The neural pathway that carries light signals from intrinsically photosensitive retinal ganglion cells to the suprachiasmatic nucleus.
Source Hattar et al., 2002.
- Photoentrainment
The process by which light synchronises the internal circadian clock to the external day and night cycle, mediated by melanopsin signalling in intrinsically photosensitive retinal ganglion cells.
Source Berson et al., 2002.
- Melanopic equivalent daylight illuminancemEDI
A standardised measure of how strongly a light source stimulates the melanopsin response, defined by the CIE system for ipRGC influenced responses to light.
Source CIE S 026/E:2018.
- Action spectrum
A curve describing the relative effectiveness of different wavelengths of light in producing a biological response. The human melatonin suppression action spectrum peaks near 460 nanometres.
Source Brainard et al., 2001; Thapan et al., 2001.
- Spectral notch filter
An optical filter that attenuates a narrow band of wavelengths while transmitting neighbouring bands, used to target specific portions of the visible spectrum.
Source Lucas et al., 2014.
- Melatonin suppression
The reduction of melatonin secretion caused by light exposure. It is strongest for short-wavelength light near 460 nanometres reaching the circadian system in the evening.
Source Brainard et al., 2001; Thapan et al., 2001.
- Circadian phase
The timing of the internal circadian clock relative to the external day. Evening short-wavelength light can delay circadian phase by suppressing melatonin.
Source Lucas et al., 2014.
References
- Hattar S, Liao H, Takao M, Berson DM, Yau K (2002). Science 295:1065 to 1070. Melanopsin-containing retinal ganglion cells project to the suprachiasmatic nucleus.
- Berson DM, Dunn FA, Takao M (2002). Science 295:1070 to 1073. Phototransduction by retinal ganglion cells that set the circadian clock. Intrinsic response peaks near 480 nm.
- Provencio I et al. (2000). Identification of melanopsin in the human retina.
- Brainard GC et al. (2001). Journal of Neuroscience 21(16):6405 to 6412. Action spectrum for melatonin regulation in humans, peak sensitivity in the 446 to 477 nm region.
- Thapan K, Arendt J, Skene DJ (2001). Journal of Physiology 535(1):261 to 267. Human melatonin suppression action spectrum, peak near 459 to 460 nm.
- Lucas RJ et al. (2014). Trends in Neurosciences 37(1):1 to 9. Measuring and using light in the melanopsin age. Melanopsin sensitivity standardised at 480 nm.
- CIE S 026/E:2018. CIE system for metrology of optical radiation for ipRGC-influenced responses to light. Defines melanopic equivalent daylight illuminance.