From the Guidelines
The best time to expose your skin to sunlight for vitamin D3 synthesis is during midday hours, typically between 10 AM and 3 PM, when UVB radiation is strongest. Aim for about 10-30 minutes of sun exposure several times per week, with arms, legs, or back uncovered (without sunscreen on these areas during this brief exposure) 1. People with darker skin may need longer exposure times than those with lighter skin. Seasonal variations matter too - summer months provide more effective UVB rays for vitamin D production than winter months, especially in locations further from the equator. This timing recommendation maximizes vitamin D synthesis because UVB rays, which trigger vitamin D production in the skin, are most abundant when the sun is highest in the sky. During early morning or late afternoon, the sun's rays pass through more atmosphere, filtering out much of the UVB radiation needed for vitamin D synthesis. Some key points to consider:
- For light-skinned individuals, sun exposure of the face and arms for just 15 minutes per week may produce tens of thousands of units of cholecalciferol 1.
- Ultraviolet B exposure may also increase vitamin D levels; however, several variables (such as the time of day, season, cloud cover, skin pigmentation, and sunscreen use) can affect the length of exposure needed to attain sufficient vitamin D levels 1.
- Remember that after your vitamin D exposure time, you should apply sunscreen to prevent skin damage and reduce skin cancer risk. It is essential to weigh the benefits of vitamin D synthesis against the risks of skin cancer associated with UVB radiation, and to consider individual factors such as skin pigmentation and geographic location when determining the optimal time and duration of sun exposure.
From the Research
Optimal Time for Sunlight Exposure to Synthesize Vitamin D3
- The best time for sunlight exposure to synthesize vitamin D3 is during the midday lunch hour, as this allows for casual sun exposure with no risk of erythema, for all latitudes some of the year, and for all the year at some (low) latitudes 2.
- Exposure to sunlight at close to solar noon is also effective in enhancing vitamin D synthesis without increasing the risk of photoaging and non-melanoma skin cancer 3.
- Season, latitude, time of day, skin pigmentation, aging, and sunscreen use all influence the cutaneous production of vitamin D3, with summer months being the most effective time for vitamin D synthesis 4, 5, 6.
- A 30-min bout of sun exposure at close to solar noon can increase serum D3 concentration in both younger and older adults, with a trend for a difference in D3 between cohorts 3.
Factors Affecting Vitamin D3 Synthesis
- Skin type, with more restrictive latitudinal and seasonal limits on adequate vitamin D production for skin type 5 than skin type 2 2.
- Age, with D3 production decreasing by 13% per decade 3.
- Clothing style, with a large influence on vitamin D production, and sunscreen use, which decreases but does not abolish vitamin D production in the skin 6.
- Latitude, with lower latitudes having more favorable conditions for vitamin D synthesis throughout the year 2, 5.