Can you get vitamin D late in the day? The science behind sun exposure timing

January 3, 2026 •

5 min read

An estimated 42% of U.S. adults are vitamin D deficient, highlighting the complexity of proper absorption beyond simple sun exposure. This raises the question for many: can you get vitamin D late in the day, or is midday the only effective time for synthesis?

Quick Summary

Vitamin D production from sunlight is highly inefficient late in the day because the sun's low angle filters out essential UVB rays. Midday is the optimal time for synthesis, but various factors influence this process, making supplements and fortified foods reliable alternatives.

Key Points

Midday is Optimal: The most effective time for your body to produce vitamin D from sunlight is midday, roughly between 10 a.m. and 3 p.m., when UVB rays are strongest.
Late Day is Ineffective: Getting vitamin D late in the day is not possible from sun exposure because the sun's low angle filters out the necessary UVB rays.
Use the Shadow Rule: A simple indicator for sufficient UVB is if your shadow is shorter than your height; otherwise, vitamin D synthesis is poor.
Consider Alternative Sources: If sun exposure is limited, rely on vitamin D supplements or fortified foods like milk, cereals, and fatty fish to meet your needs.
Account for Skin Tone and Location: Darker skin and higher latitudes significantly reduce vitamin D production from sunlight, necessitating longer exposure or alternative intake.
Prioritize Safety: For any sun exposure, brief periods are sufficient for vitamin D. For longer durations outdoors, use sunscreen to protect against skin cancer.

The Science Behind Vitamin D Synthesis and Sunlight

Vitamin D is often referred to as the "sunshine vitamin" because our skin can produce it when exposed to sunlight. Specifically, it is the sun's ultraviolet B (UVB) rays that trigger the conversion of a cholesterol precursor in the skin into vitamin D3. The amount of UVB radiation that reaches the Earth's surface, and subsequently your skin, is not constant throughout the day. It is heavily influenced by the angle at which the sun's rays enter the atmosphere.

When the sun is low on the horizon, as it is in the early morning and late afternoon or evening, its rays travel through a greater thickness of the Earth's atmosphere. This atmospheric layer acts as a filter, scattering and absorbing a significant portion of the UVB radiation. As a result, the sunlight you receive during these times is predominantly composed of ultraviolet A (UVA) rays, which are not effective for vitamin D production. The necessary UVB rays are most intense when the sun is highest in the sky, during the midday hours, typically between 10 a.m. and 3 p.m..

The “Shadow Rule” Explained

For those wondering if the time is right for vitamin D synthesis, the "shadow rule" offers a simple visual guide. If your shadow is shorter than your height, the sun is high enough in the sky for the UVB rays to be potent. Conversely, if your shadow is longer than you are tall, the sun's angle is too low, and vitamin D synthesis is significantly diminished or non-existent. This rule accounts for seasonal and geographical variations, making it a useful tool for gauging the effectiveness of your sun exposure in real-time.

Factors That Influence Vitamin D Production

Several variables affect how much vitamin D your skin can produce from sunlight. It's not as simple as just stepping outside. The following factors must be considered:

Time of day and year: As discussed, midday is most effective. The summer months are also far more productive for vitamin D synthesis than the winter months, especially at higher latitudes.
Latitude: People living farther from the equator receive less direct sunlight and weaker UVB rays throughout the year. They may not produce any vitamin D from sunlight for several winter months and will require alternative sources.
Skin color: Melanin, the pigment that determines skin color, acts as a natural sunscreen. Individuals with darker skin tones have more melanin, which absorbs UVB radiation and slows down vitamin D production. This means they need more sun exposure time than those with lighter skin to produce the same amount of vitamin D.
Age: The skin's ability to synthesize vitamin D decreases with age. Older adults may need more time in the sun or rely more heavily on supplements to maintain adequate levels.
Sunscreen and clothing: Sunscreens with an SPF of 8 or higher significantly block the UVB rays needed for vitamin D synthesis. Similarly, clothing that covers the skin will prevent any vitamin D production in those areas.

How UVB Absorption Decreases Later in the Day

During the early morning and late evening, the sun's rays travel through the atmosphere at a much steeper angle, increasing the atmospheric filtering of UVB radiation. This means that even though you can feel the sun's warmth, the specific wavelength needed for vitamin D production is largely absent. While late afternoon sun exposure can still be beneficial for mood and for getting some UVA exposure, it is a very inefficient method for increasing your body's vitamin D stores. Relying on late-day sun for vitamin D can lead to deficiency, especially in individuals with other limiting factors like darker skin or northern latitude.

Alternative Sources for Vitamin D

For those unable to get adequate sun exposure, whether due to location, skin sensitivity, or time of day, there are reliable alternatives. These include dietary intake and supplementation, which are crucial for maintaining healthy vitamin D levels.

Fatty Fish: Excellent sources include salmon, mackerel, and tuna.
Fortified Foods: Many common food products, such as milk, orange juice, and breakfast cereals, are fortified with vitamin D.
Egg Yolks: A smaller but still beneficial source of the vitamin.
Cod Liver Oil: A concentrated dietary source.
Supplements: Vitamin D supplements are widely available and can be a very effective way to ensure adequate intake, especially during months with limited sunlight. The D3 form is generally more readily absorbed by the body than D2.

Learn more about dietary sources of vitamin D by visiting the National Institutes of Health Fact Sheet.

Sun Exposure Timing Comparison


Feature	Midday Sun Exposure (Approx. 10 a.m. - 3 p.m.)	Late Afternoon/Evening Sun Exposure
UVB Intensity	Highest; sun is highest in the sky.	Lowest; atmospheric filtering is highest.
Vitamin D Synthesis	Most efficient and effective.	Ineffective; primarily UVA rays.
Time Required	Minimal time needed (e.g., 10-15 minutes).	Significantly longer exposure, but ultimately futile for vitamin D.
Risk of Sunburn	Higher risk, requires careful time management.	Lower risk of sunburn due to lower UVB intensity.
Overall Benefit	Optimal for natural vitamin D production.	Less beneficial for vitamin D, but still provides light exposure.

Balancing Sun Exposure and Safety

While midday sun is key for vitamin D, caution is essential. Brief, unprotected sun exposure is sufficient for most people, and prolonged time in intense sunlight increases the risk of skin cancer. The goal is to maximize vitamin D production while minimizing the risk of skin damage. For longer outdoor periods, sunscreen and protective clothing are crucial for safety after your brief, intentional vitamin D exposure. Individuals should be mindful of their skin type and always avoid sunburn.

Conclusion

In summary, it is largely ineffective to get vitamin D late in the day from sun exposure. The low angle of the sun at these times means the atmosphere filters out the vital UVB rays needed for skin synthesis. For optimal natural vitamin D production, brief, regular midday sun exposure is the most efficient method, especially during the sunnier seasons. However, the best approach for maintaining healthy vitamin D levels is often a combination of strategic, safe sun exposure and consistent use of supplements or fortified foods, particularly for those with darker skin, limited mobility, or who live in higher latitudes. Consulting a healthcare provider for personalized advice and dosage recommendations is always the best course of action.

Frequently Asked Questions

Neither early morning nor late evening sun is effective for vitamin D synthesis. At these times, the sun's angle is too low, and the atmosphere blocks the necessary UVB rays, leaving only UVA, which does not produce vitamin D.

No, people with darker skin already need significantly more midday sun exposure to produce vitamin D because of their higher melanin content. Late-day sun, which lacks UVB, is not an effective source for anyone, especially those with darker skin.

No, glass blocks the sun's UVB rays, so sitting next to a sunny window does not allow for vitamin D synthesis. You must have direct skin exposure to unfiltered sunlight.

During winter, especially at higher latitudes, sun exposure is insufficient for vitamin D production. The best way to get vitamin D is through supplements and fortified foods.

Yes, sunscreen with an SPF of 8 or more blocks the UVB rays required for vitamin D synthesis. If you choose to get vitamin D from sun exposure, you need a brief period without sunscreen, but always be mindful of sun safety and avoid burning.

Common symptoms of low vitamin D can include fatigue, bone pain, muscle weakness, and frequent infections. A blood test from a healthcare provider is the most accurate way to check your vitamin D levels.

Research on this is mixed, but some studies suggest that taking vitamin D supplements too close to bedtime could interfere with melatonin production and disrupt sleep. It is generally recommended to take it earlier in the day if you find it affects your sleep patterns.

This is a common misconception. The most effective period for vitamin D synthesis is midday when the sun is highest, not early morning or late afternoon. However, some experts suggest early morning sun might be slightly better than evening sun in some regions due to lower pollution levels.