Your circadian clock is a 24-hour molecular oscillator in nearly every cell. Light is its primary zeitgeber — the force that keeps it aligned with the solar day.
In 2002, Provencio discovered melanopsin — a photopigment in a small subset of retinal ganglion cells (ipRGCs). These cells do not help you see images. They measure ambient light and send a direct signal to the suprachiasmatic nucleus, the brain's master clock.
The ipRGC action spectrum peaks near 480 nm — cyan blue. A 470 nm LED at ~100 lux is roughly 100 melanopic-EDI, enough to shift phase. This is the basis for the CIE S 026:2018 standard used throughout SunCycle.
Not all light is equal for circadian entrainment. Here are the population-level bands SunCycle uses to guide recommendations.
Windowless room or dim home. Mel-EDI under 50.
Typical home/office. Mel-EDI 50-200. Too low to entrain.
Well-lit office or sunny window seat. Sub-entraining.
Cloudy day outside. Strongly entraining.
Open shade. Strong circadian signal.
Sun on skin. Maximum entrainment; mel-EDI 10,000+.
The suprachiasmatic nucleus (SCN) sits at the base of the hypothalamus, just above the optic chiasm. It receives direct input from ipRGCs and synchronizes peripheral clocks in the liver, muscle, and fat tissue — all via the autonomic nervous system and hormonal signals.
Morning light advances the clock (makes you sleepy earlier). Evening light delays it (pushes sleep later). The phase-response curve (PRC) is well-characterized: 30 minutes of 1,000+ lux within 1h of waking produces the strongest advance, while even dim blue light after sunset can delay sleep by 40+ minutes.
See how these principles translate into a personal light schedule for your location.
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