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Why "20 minutes" is the wrong answer, and what actually determines your vitamin D from the sun — UV, skin type, exposed skin, and season.

The single most reliable thing light does for you: anchoring your body clock so you fall asleep and wake more easily. What the evidence says, and how to do it.

Above a certain latitude, the winter sun physically can't make vitamin D — no matter how long you stay out. Here's why, and what to do instead.

In controlled testing SPF 8 essentially abolished synthesis — but real-world application is a fraction of the test thickness. The honest answer cuts both ways, and what actually controls your vitamin D sits well above sunscreen on the list.

UV Index 3 is the working threshold for making vitamin D. Why that exists, the shadow rule that checks it without an app, and where the number misleads.

A pill reliably raises your blood level; sunlight does that and several things a capsule cannot. What VITAL, the largest trial, actually found.

The guides above are the short version. This is the long one: a 19,000-word cited review of the whole literature — vitamin D as a hormone, why the official targets are disputed, the red and near-infrared half of sunlight, the body clock, the condition-by-condition evidence map — with a grade on every claim. See the science library.