Your body can make vitamin D out of sunlight, a fact you may already know. Unfortunately, however, it’s not as easy as it sounds. The amount of vitamin D produced from sunlight depends on the time of year, time of day, where you live in the world, the color of your skin, how much skin is exposed, whether you are wearing sunscreen or not, and your body’s ability to convert vitamin D into its active form.

Unless it’s summertime, the skin makes little to no vitamin D at latitudes above 37 degrees. See this map to find your latitude. But even south of 37 degrees, it is very common to find people with insufficient vitamin D levels. There are many factors that determine vitamin D production from sunlight. The best way to attain a healthy vitamin D status is to get your level checked regularly and supplement with vitamin D accordingly.

A recent Mayo Clinic study published in the Public Library of Science ONE (PLoS ONE) journal found that vitamin D levels in the United States peak in August and hit an all-time low in February. “In this study, we have shown that vitamin D levels lag the solar cycle, peaking in August and troughing in February,” stated Amy Kasahara, a researcher of the study. “Even with food fortification, vitamin D levels in the population show a high level of seasonality due to the influence of sunlight.”

This study further supports the need for vitamin D supplementation. Fortunately, vitamin D is inexpensive and readily available. Be sure to test your vitamin D levels regularly. For most people, a level of at least 50 ng/mL is considered healthy.