Comment by Amy Morgenstern (amy-morgenstern) on 100 Tips for a Better Life · 2020-12-26T18:55:58.799Z · LW · GW

Vitamin D deficiency is incredibly easy to remedy: go outside. This is also good advice for everything from boredom to nasal congestion to depression.

How much time you need outside depends on the level of melanin in your skin, the amount of cloud cover, and the time of day, but it is very low. 

(I am also not a doctor.)