Can You Get Enough Sun in Winter? What to Know About UVB and Vitamin D

When clocks fall back for the end of daylight saving time, many people start commuting in the dark and spending daylight hours indoors. According to Chris Brooks, chief strategy officer at Solius Labs, which makes an at-home UVB light therapy device, winter also brings a less obvious change: the part of sunlight the body uses to make vitamin D nearly disappears.

“We think of sunlight as one of the foundational pillars of health, alongside nutrition, exercise and sleep,” Brooks said. “In winter, that pillar quietly goes missing.”

Winter sunlight is missing a key ingredient

UVB is the part of sunlight that triggers vitamin D production in the skin. In winter, the sun sits lower in the sky, so its light travels through more atmosphere, and the ozone layer filters out most of the UVB before it reaches the ground, Brooks said.

• North of about the 35th parallel, roughly the latitude of Memphis, Tennessee, or Albuquerque, New Mexico, winter sunlight provides little to no usable UVB.
• In cities like Seattle or Boston, that gap can last four to five months.
• Even in summer, UVB peaks for only two to three hours around midday, when many people are indoors.

Brooks sees it every year in the Pacific Northwest, when people head outside on a rare sunny January day.

“What they’re actually getting is UV risk with almost none of the UVB benefit,” he said.

UVA versus UVB: what’s the difference?

Ultraviolet light comes in three types. UVC is blocked by the atmosphere. The other two reach the ground and behave very differently, Brooks said.

UVA

• Makes up most of the UV that reaches Earth
• Stays fairly constant throughout the day and year
• Passes through window glass
• Penetrates deeper into the skin and drives much of the long-term skin damage, including photoaging
• Is the main type emitted by tanning beds

UVB

• Makes up less than 1% of the sun’s energy that reaches Earth
• Is absorbed in the outer layer of skin, where it starts vitamin D production
• Is blocked by glass, so sitting by a sunny window provides UVA exposure but not UVB

Who gets less UVB?

Several factors affect how much UVB the body can use, Brooks said:

• Location: The farther from the equator, the less UVB is available.
• Time of day: UVB is strongest only around midday.
• Amount of exposed skin: A few minutes of sun on the face and hands covers too little skin to make much difference.
• Skin tone: Melanin absorbs UVB, so people with darker skin need more exposure for the same effect.
• Age: Older adults produce substantially less vitamin D from the same sun exposure.

Sunlight versus supplements

Supplements can raise blood vitamin D levels, Brooks said, but absorption depends on digestion and varies with genetics, body weight and gut health. People with conditions such as Crohn’s disease or celiac disease may absorb less.

Brooks noted that large observational studies link more sun exposure and higher vitamin D levels with lower rates of several diseases, but many of those benefits have not held up in randomized trials of vitamin D pills.

“A growing view among researchers is that vitamin D is a biomarker of sun exposure, but not the single driver of many of the sun’s health benefits,” he said.

He added that vitamin D made in the skin has a built-in safeguard. Once the skin has made enough, it converts the excess into inactive compounds, so sunlight can’t cause the toxic vitamin D buildup that very high-dose supplements can.

Not all experts agree on how to get vitamin D. The American Academy of Dermatology recommends getting it from food, fortified products and supplements rather than unprotected UV exposure, because UV radiation is a known skin cancer risk factor. The academy says no proven safe level of sun or tanning exposure maximizes vitamin D without raising skin cancer risk.

Finding the right dose

Brooks said the balance between UVB’s benefits and its risks comes down to dose. Too little UVB provides no benefit, while too much causes sunburn, damages skin DNA and raises skin cancer risk. The right amount depends on skin tone, age, location, season, time of day and recent sun exposure, which is why one-size-fits-all advice falls short.

“For one person it’s too little to matter, and for another it’s enough to burn,” he said.

UVB light therapy has long been used by dermatologists to treat skin conditions in a clinic. In January 2026, the FDA cleared Solius Pro, an over-the-counter UVB light panel intended to stimulate vitamin D production in adults 22 and older. The device uses a skin sensor to set a personalized dose, and the company recommends weekly sessions of under five minutes.

Winter sunlight tips

• Get outside around midday when you can, since that’s when UVB is strongest.
• Remember that sunlight through a window provides UVA but not UVB.
• Protect your skin from burning, even in winter, especially at higher elevations or around snow.
• Ask your health care provider whether a vitamin D blood test makes sense, especially if you have darker skin, are older or have a digestive condition.
• Talk with your provider or a dermatologist before starting supplements or any UV light therapy.