Above the 36 parallel you can't absorb Vitamin D from the sun. Checked: sun must be above an angle of 50° from the horizon in order for UVB to penetrate the earth’s atmosphere. Other wise it bounces. Which you need 20 to 30 minutes of sun exposure in the early afternoon. Check your area. (Enter your lat/log/time) (note: Miami Florida January and February is under 50 deg) https://www.esrl.noaa.gov/gmd/grad/solcalc/azel.html