Circadian Sleep

UV eye protection and circadian light input are not mutually exclusive — but the timing, type, and coverage of protection determine whether you preserve both. Wide-brim hats reduce facial UV by up to 76% but cannot block reflected UV from below. Wrap-around sunglasses provide the best geometric coverage. UV-blocking contact lenses close the gaps that […]

Yes, the phase advance is measurable — and driven by specific changes in the brain and eye, not habit. With age, the suprachiasmatic nucleus (SCN) loses VIP neurons that sustain circadian amplitude, the crystalline lens yellows and blocks the blue-wavelength light that resets the master clock, and melatonin amplitude declines from the third decade onward.

Your circadian clock depends on approximately 4,000-5,000 melanopsin-containing retinal ganglion cells (mRGCs) per eye. After age 50, these cells begin losing their dendritic architecture — the branching structures that gather light and relay it to the brain’s master clock. After 70, cell death accelerates: 31% fewer mRGCs remain compared to midlife. In Alzheimer’s disease, amyloid-beta

Yes — but not the master clock. Meal timing is a zeitgeber for peripheral organs including the liver, kidney, and adipose tissue, but it does not move the brain’s suprachiasmatic nucleus (SCN). When you eat at odds with your light-dark cycle, your peripheral clocks move while your SCN holds its position — internal circadian misalignment.

Aging eyes do receive less UV because the lens yellows and the pupil shrinks. But retinal melanin — the eye’s built-in UV shield — declines 2.5-fold and reverses from antioxidant to pro-oxidant. Aqueous humor vitamin C declines with age. Lens glutathione falls from over 20 mM to under 3 mM. The light that gets through

It depends on two variables the blanket advice omits: strain intensity and chronotype. The largest study ever conducted on evening exercise and sleep — tracking 14,689 people across 4 million nights — found that high-strain exercise finishing at least four hours before sleep causes no meaningful disruption, while maximal-strain exercise within two hours delays sleep

Deep learning models trained on retinal photographs can estimate how old the body is — independent of chronological age — with a mean error of under 3 years. When the predicted retinal age exceeds chronological age, each 1-year gap is associated with a 2% increase in all-cause mortality risk. The retinal age gap reflects vascular,

At least six circadian mechanisms change after 40: the SCN loses amplitude as VIP neurons decline, melatonin production drops from pineal calcification, the cortisol rhythm advances earlier, the core body temperature rhythm flattens, lens yellowing reduces blue light reaching the retina, and peripheral clocks become harder to re-entrain through exercise. Some of these are structural

Several structures in your eye cannot repair or regenerate themselves. Corneal endothelial cells decline from approximately 5,000 per mm² at birth to a decompensation threshold near 500 cells per mm² — and every UV exposure damages cells that cannot be replaced. Retinal melanin declines 2.5-fold over a lifetime, and the remaining melanin undergoes photooxidation, reversing

A night owl preference and delayed sleep phase disorder share an evening chronotype, but they are not the same condition. Delayed sleep phase disorder is a circadian rhythm disorder: a clock delay persisting three or more months with functional impairment — difficulty meeting social and work demands, daytime fatigue, and elevated mood and anxiety disorder