What time of day is best for vitamin D?

Bottom lineMidday, roughly 10 am to 3 pm, is when UV-B reaches the ground and vitamin D synthesis actually happens - 10 minutes at noon beats an hour at 8 am. Keep midday windows short and well under your burn time, and remember that in high-latitude winters no time of day produces much vitamin D.

The middle of the day - roughly 10 am to 3 pm, when the sun is highest. That is when UV-B, the wavelength your skin uses to make vitamin D, actually reaches the ground in force.

Why midday wins

UV-B is filtered heavily by the atmosphere when the sun sits low. Early morning and evening sun is mostly UV-A and visible light: pleasant, but nearly useless for vitamin D. The practical consequence surprises people - 10 minutes at noon beats an hour at 8 am for vitamin D in most seasons and latitudes.

This is also why the common advice to "get your sun before 10 or after 4" is really sun-safety advice, not vitamin D advice. It minimizes burn risk by minimizing UV-B - which minimizes vitamin D at the same time.

Resolving the midday dilemma

The efficient pattern is short and deliberate:

• Check the UV index first. At UV 3-6, a brief midday window is productive and manageable; at UV 8+, your window shrinks to minutes.
• Expose decent skin area (arms and legs beat face and hands), stay well under your time-to-burn, then cover up or apply sunscreen.
• Darker skin tones need meaningfully longer; very fair skin needs less time and more caution.

In winter at higher latitudes, even midday sun may sit below UV 3 for months - in that case no time of day rescues synthesis, and food or a supplement carries you to spring.

Femora's Sunshine feature shows your live personal rate (IU per minute) and a burn countdown, so "is right now worth it?" stops being a guess.

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