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A roughly 24-hour cycle that regulates sleep, alertness, hormone release, and body temperature. The brain's master clock — the suprachiasmatic nucleus — synchronizes this rhythm primarily through light reaching the eyes. Without daily light cues, the clock drifts.

• Sleep–wake timing
• Daily cortisol curve
• Body temperature variation
• Hormonal cycles

A photopigment found in a small population of retinal ganglion cells (ipRGCs). Most sensitive to short-wave blue light around 480nm. Unlike rod and cone pigments, melanopsin doesn't contribute to vision — it sends timing signals to the master clock. Its discovery in 2000 reshaped circadian science.

• Peak sensitivity at 480nm
• Signal independent of conscious vision
• Most active in morning light
• Found in ipRGC cells

One lux equals one lumen per square meter. Lux measures how much visible light is hitting a surface — not the source. A well-lit office is ~500 lux. A cloudy day outdoors: 10,000 lux. Direct sunlight: 100,000+. The differences are why "well-lit indoors" rarely matches "outdoors."

• Candlelight ~1 lux
• Office ~300–500 lux
• Overcast day ~10,000 lux
• Clear noon ~100,000 lux

Measured in Kelvin (K). A theoretical "black body" radiator at that temperature emits the same color as the light source. Lower numbers (2000K) are warm and red-shifted. Higher numbers (6500K+) are cool and blue-shifted. The daily solar arc moves continuously across this scale.

• Candle ~1800K
• Incandescent ~2700K
• Daylight ~6500K
• Clear blue sky ~10,000K

Wavelengths roughly 420–490nm. The most circadian-active portion of the visible spectrum because melanopsin peaks here. Beneficial in the morning (anchors your rhythm), problematic at night (suppresses melatonin). It's not the light that's bad — it's the timing.

• Anchors morning circadian rhythm
• Suppresses evening melatonin
• Concentrated in screens and LEDs
• Naturally absent at sunset

The brief window when the sun is below the horizon but its light still reaches the upper atmosphere. Three phases — civil, nautical, astronomical — defined by the sun's angle below the horizon. The spectrum is distinctly violet-blue from high-altitude scattering. Lasts 20–40 minutes.

• Civil: sun 0–6° below
• Nautical: 6–12° below
• Astronomical: 12–18° below
• Violet-blue dominant spectrum

The period shortly after sunrise or before sunset when the sun is low. Light travels through many times more atmosphere than at noon, scattering away blue wavelengths and enriching reds, oranges, and ambers. The body reads this as "the day is ending."

• Lasts ~40 minutes
• Solar elevation below 6°
• Spectrum heavily red-shifted
• Begins melatonin preparation

Intrinsically photosensitive retinal ganglion cells. A small population — about 1% of retinal ganglion cells — that contain melanopsin and respond directly to light. Their signal bypasses vision and goes to the brain's master clock, the SCN. The whole concept of "non-visual light" depends on them.

• ~1% of retinal ganglion cells
• Discovered in 2000
• Project to suprachiasmatic nucleus
• Drive pupil response in dim light