Many people have no idea that winter sun = no vitamin D....lets discuss

As the “ber months” arrive (in the Northern hemisphere), something major shifts in your biology: UVB light - the part of the spectrum that makes vitamin D in your skin - goes away for a number of months.

And here’s the truth: winter health is not just about popping a vitamin D pill...(​click here to read my article on vitamin D supplements & precautions​).

Staying healthy & avoiding seasonal depression, weight gain and sleep issues largely comes down to understanding how to live seasonally AND take advantage of the sunlight you do have available.

Upcoming articles will focus on fall/winter health - nutrition & hydration, and today I want to explain why the benefits of sunlight don't go away due to losing UVB.

Keep reading for the article with citations....

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To understand your UVB window - check the UV index & understand when UVB returns for you - make sure you have downloaded ​MyCircadianApp​. The free version will give you all this information, and you can try the premium version with code SARAHK.

Who Actually Loses UVB?

• Around 35° latitude (roughly Los Angeles, NC/TN border, southern Spain, southern Japan) and northward, UVB rays (290–315 nm) become insufficient for vitamin D production in winter.
• By ~42° latitude (Boston, Rome, Chicago, Beijing), vitamin D production essentially shuts off for 4–5 months.
• At 50° latitude and higher (London, Berlin, Calgary), you can go 6+ months with no usable UVB.
• At 60° latitude and higher (Stockholm, Helsinki, Alaska), it can be 7–8 months of no UVB-driven vitamin D synthesis *and no UVA for some parts of the year (more on that later)*

What Months is UVB gone?

• 30° latitude (Atlanta, Houston, Cairo, Florida): Usually maintain vitamin D synthesis year-round, though efficiency is lower in winter (more info in the next bullet)
• 35°–40° latitude (Denver, Madrid, Tokyo): Vitamin D winter lasts ~2–4 months (Nov–Feb).
• 40°–45° latitude (New York, Milan, Toronto): Vitamin D winter lasts ~4–5 months (Nov–Mar).
• 50°–55° latitude (London, Berlin, Vancouver): Vitamin D winter lasts ~5–6 months (Oct–Mar).
• 60° latitude+ (Oslo, Helsinki, Alaska): Vitamin D winter can last 7–8 months (Sept–Apr).

What UVI Is Needed to make Vitamin D?

• The often-quoted rule is that you need a UVI of ~3 or higher to make vitamin D -but that’s only a general population guideline.
• Lighter skin (Fitzpatrick I–II, northern haplotypes) can sometimes make vitamin D at UVI 2–3, since less melanin filters UVB.
• Darker skin (Fitzpatrick IV–VI, equatorial haplotypes) usually requires UVI 4–5+ and longer exposure, because melanin naturally blocks UVB.
• Latitude & solar angle matter: At higher latitudes, even with UVI ~3, the low sun angle in winter means almost no usable UVB reaches the skin. Midday exposure is essential.
• Individual biology matters: Age, body composition, and mitochondrial haplotype all shift how efficiently you convert sunlight to vitamin D.

At the end of the day - UVI 3 is a rough marker, but whether that’s “enough” depends on your skin, your ancestry, and your environment.

Key Reference Points:

• Webb AR, Kline L, Holick MF. Annu Rev Nutr. 1988. Classic study that mapped vitamin D winter by latitude.
• Holick MF. J Clin Invest. 1981. Demonstrated the role of zenith angle of the sun in vitamin D production.
• Engelsen O. J Nutr. 2010. Detailed modeling of UVB and vitamin D synthesis worldwide.

 

The good news is that vitamin D is only one piece of the sunlight puzzle. Even when UVB disappears in the winter, other wavelengths like UVA, near infrared (NIR), and far infrared (FIR) continue to reach the earth’s surface year-round.

These crucial light signals help to regulate appetite, stabilize mood, protect mitochondria, and even help conserve the vitamin D you already have.

Sunlight is a full-spectrum therapy - which means your biology still has powerful tools to thrive in the darkest months if you know how to use them. (Let's dive in ⬇️⬇️) 

UVA (315- 400 nm) is your hidden ally in winter, because unlike UVB, it’s still abundant across most latitudes.

The eyes are especially sensitive to UVA: through the retina, it boosts serotonin and dopamine, which stabilize mood, appetite, and energy, and the serotonin you make during UVA is recycled into that night's melatonin (so your morning light exposure is crucial for good sleep that night).

Most people don’t realize that UVA light doesn’t only work on the skin - it also activates POMC neurons in the brain through the eyes. This matters because POMC controls the release of peptides like α-MSH and β-endorphin, which regulate appetite, mood, and energy balance.

In other words, when you step outside in morning light (without sunglasses or glass in the way), you’re sending a signal to both your skin and your brain that stabilizes hormones and helps your biology sync to the season.

And in places like Norway, Sweden, and other high latitudes, UVA itself disappears for part of the year - which is why those populations experience even bigger seasonal shifts. That’s when red and infrared light (available year-round) become essential supports for your mitochondria and circadian system.

Use ​MyCircadianApp ​to find out when UVA happens for you!

 

One of the most powerful daily resets for your brain and hormones is morning light exposure.

Outdoor morning light typically measures between 1,000 -100,000 lux, compared to just 100- 300 lux indoors - a dramatic difference your circadian system can clearly detect.

Studies show you need at least 500 lux (which is equivalent to those dark, rainy winter mornings) in the morning to properly align metabolism, appetite, and mood.

When combined with UVA, morning light boosts serotonin and dopamine production, which in turn regulates hunger hormones like ghrelin and leptin.

Without this signal, your brain interprets the day as “permanent dusk,” driving cravings, fatigue, low energy, and increased risk of seasonal depression.

This is why stepping outside in the morning - even for just a few minutes - is one of the most effective, science-backed tools for preventing mood crashes and metabolic disruption in the darker months

Remember to use ​MyCircadianApp ​to measure your lux (and don't forget to keep lux below 10 the 3 hours leading up to bed to protect your melatonin)

(Cajochen et al., 2005; Partonen & Lönnqvist, 1998). 

Near infrared (NIR) light is unique because it penetrates deeply into tissues, where it stimulates the production of mitochondrial melatonin.

Unlike the pineal melatonin that rises at night, this form is made locally inside cells to protect mitochondria from oxidative stress.

During low-UV seasons like winter, NIR becomes especially valuable: it strengthens cellular defenses, supports mitochondrial efficiency, and even helps buffer the immune system when vitamin D levels are low.

Importantly, NIR is present year-round - even on cloudy days or through snow - making it one of the most reliable forms of light to keep your biology resilient in the darker months

(Reiter et al., 2016). 

Far infrared (FIR) light plays a unique role in structuring exclusion zone (EZ) water - a fourth phase of water that forms around proteins and membranes in the body.

EZ water acts like a cellular battery, storing solar energy and improving efficiency of biological processes, and in the winter, when UVB-driven vitamin D production is low, EZ water can act as an alternative energy reserve to help keep cells functioning optimally.

The best part? FIR penetrates deeply and is available all year long, even on cloudy days or through snow, making it a consistent signal to support your biology in the darker months.

(Pollack, 2013).

 

Cold exposure is more than just a “stress” - it’s a powerful seasonal signal that tells your body to adapt to winter conditions.

Research shows that cold can stimulate mitochondria to release biophotons (ultra-weak light emissions from cells), which may help preserve vitamin D stores and support cellular communication during times of sunlight scarcity.

This process reflects how your biology is designed to work with seasonal cues: when UVB and vitamin D production drop, cold steps in as a complementary stimulus to strengthen immunity, build resilience, and optimize energy balance.

Instead of being something to fear, cold is actually a natural adaptation tool that helps your body thrive in darker, colder months.

The good news? If you live in a very cold climate - all you need to do is go outside to get your cold therapy!

(van Wijk et al., Photochem Photobiol Sci.).

 

Your vitamin D levels are actually designed to dip slightly in the winter - it’s part of circannual biology, the yearly rhythm your body follows with the seasons.

The problem isn’t the dip itself, but the fact that modern life accelerates it: Spending all day indoors, under artificial light that lacks UVA, NIR, and FIR, while being blasted with blue light at night, pushes your system further out of sync than nature ever intended.

Instead of a mild seasonal adjustment, you end up with a sharper vitamin D deficiency, compounded by mood swings, metabolic slowdown, and cravings. In other words, it’s not winter that’s the problem - it’s how disconnected we’ve become from the seasonal signals that are supposed to guide us.

How to address this ⬇️⬇️

 

How to Apply This....

Here’s how you can stay aligned this winter:

 ​Download MyCircadianApp​ - Track your sunrise & UVA windows, measure lux, and learn how to time your light exposure so your circadian rhythm and hormones stay on track. Use code SarahK to try the premium version for free.

Get the Quantum Winter Blueprint (​click here​) - Winter doesn’t have to mean weight gain, fatigue, and depression. The Quantum Blueprint has 4 fall/winter courses to give you everything you need to feel your best!

The Leptin Master Plan ​(click here) ​- We still have 5 live calls left, and you can still join. If you want a practitioner-level roadmap to reset hormones, master circadian biology, and finally fix metabolism at the root, this is your chance to get direct guidance in real time.

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Citations:

• Webb AR et al. Annu Rev Nutr. 1988.
• Lambert GW et al. Lancet. 2002.
• Cajochen C et al. J Clin Endocrinol Metab. 2005.
• Partonen T, Lönnqvist J. Lancet. 1998.
• Reiter RJ et al. Exp Gerontol. 2016.
• Pollack GH. The Fourth Phase of Water. 2013.
• van Wijk EPA et al. Photochem Photobiol Sci