// LIGHT & CIRCADIAN BIOLOGY

Light & Circadian Biology

Photoreceptors, melanopsin, SCN master clock, cortisol/melatonin rhythms, blue light

Your body runs on a 24-hour clock controlled by the suprachiasmatic nucleus (SCN), and light is the primary input that sets it. Non-visual photoreceptors in your eyes (ipRGCs) detect 480nm blue light via melanopsin and signal the SCN directly. Morning light triggers cortisol; evening darkness triggers melatonin. Modern screens flood this system with exactly the wrong wavelength at exactly the wrong time — disrupting sleep, metabolism, immunity, and repair.

24-HOUR CIRCADIAN RHYTHM6AM9AM12PM3PM6PM9PM12AM3AM6AMDAYNIGHTCortisol awakening response↑50-75%DLMO · Melatonin onsetDeepest sleep · Peak melatoninLowest body temperatureCortisolMelatonin10 lux at night \u2192 50% melatonin suppression \u00B7 Gooley et al., JCEM 2011CORTISOL & MELATONIN \u00B7 OPPOSING RHYTHMS

Mechanism

MORNINGLight \u2192 retinaipRGCs (480nm)SCNCortisol \u2191EVENINGDarknessSCN \u2192 PinealMelatonin \u2191Core temp \u2193Sleep onsetGooley et al., JCEM 2011
50%

melatonin suppression from just 10 lux of light at night — dimmer than a single candle. Screens emit peak energy at 450-495nm, directly in melanopsin's sensitivity range.

Gooley et al., JCEM 2011 · Brainard et al., J Neurosci 2001

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