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The Biology of Morning Light
A small molecule called melanopsin
In short
A light sensor in your eye, found in 2000, sends morning light straight to your body clock, most strongly in the first hour after waking.
In the retina, alongside the rods and cones that build vision, lies a third class of photoreceptor discovered only in 2000: the intrinsically photosensitive retinal ganglion cell, or ipRGC. Its pigment, melanopsin, is most sensitive to short-wave blue around 480nm.
This cell does not contribute to image formation. Instead, it sends its signal directly to the suprachiasmatic nucleus — the brain's master clock. Morning light, rich in the wavelengths melanopsin reads, is what tells your biology that the day has begun. The signal is most powerful in the first hour after waking.
Sources
- A Novel Human Opsin in the Inner Retina — Provencio et al. (2000), Journal of Neuroscience
- Phototransduction by Retinal Ganglion Cells That Set the Circadian Clock — Berson, Dunn & Takao (2002), Science
- Melanopsin-Containing Retinal Ganglion Cells: Architecture, Projections, and Intrinsic Photosensitivity — Hattar et al. (2002), Science