[jessicas_rabbit]

In order to produce vitamin D, your skin needs continuous time in the sun to fully react with sunlight.

30 minutes of continuous chemical reaction will have vastly different effect than a reaction that stops & starts repeatedly, for a sum total of 30 minutes.

[cyberax]

You actually are going to synthesize _more_ vitamin D if you do fractionated exposure.

That's because vitamin D synthesis is an equilibrium reaction, UV drives backward and forward reactions. So you'll get more vitamin D if you expose your skin to the sun for a few minutes, then let the synthesized vitamin to diffuse out of the skin layer, and then expose yourself again.

[jessicas_rabbit]

That makes sense. Thanks for explaining.