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Why Psoriasis Flares in Winter — and What Actually Changes

Last revised 2026-09-05

The short answer

If your psoriasis is worse between November and March, you are not imagining it and you are not failing at anything. Four things change in winter, three of them documented in the psoriasis literature.

The UVB stops arriving. Above roughly 37° north — north of San Francisco, Denver or Richmond, Virginia — UVB is essentially absent from November through February. That removes both vitamin D production and a separate anti-inflammatory system in your skin.
Your body clock drifts. Shorter days, darker mornings and more artificial light at night disturb the circadian genes that gate how responsive your immune cells are to IL-23, the cytokine upstream of psoriatic inflammation.
Skin injury goes up. Heavier clothing, more friction, drier skin, more scratching. Psoriasis appears at sites of skin injury in up to 75% of patients, and the delay is 10 to 20 days — which is why December's friction shows up in January.
Behaviour changes. Less time outside, less exercise, disrupted sleep, more alcohol, more stress — all with documented effects on the same pathways.

The highest-leverage thing you can do costs nothing: go outside in the morning anyway. Ten to thirty minutes of outdoor light on a cloudy winter day still delivers 10,000 lux or more. Your office delivers about 500.

One honest note before the detail: Heal Psoriasis barely addresses winter as a topic. It maps "worse in winter, better in summer" to two of its fifteen root causes and moves on. What follows is assembled from chapters about light, nerves and hormesis rather than about seasons — and where the book has nothing to say, this article says so rather than inventing an answer.

1. The light disappears — and vitamin D is only half of what you lose

Chapter 7 of the book is built on a distinction almost every article about winter psoriasis misses.

Sunlight on skin does two separate things. It makes vitamin D. And it activates a cascade beginning with a protein called POMC, which your skin chops into alpha-MSH — a potent anti-inflammatory peptide — along with beta-endorphin and ACTH.

Alpha-MSH is the one that matters. It blocks NF-κB, the master switch for the IL-17, TNF-alpha and IL-6 genes, and converts aggressive effector T cells into regulatory T cells. It is your skin's own anti-inflammatory, and the switch that turns the factory on is UVB.

Which leads to the distinction: a vitamin D supplement gives you vitamin D. It does not give you alpha-MSH, beta-endorphin, circadian entrainment or the local melatonin system. Chapter 7 argues that is why vitamin D trials in psoriasis have shown mixed results — levels improve without proportional improvement in skin.

That does not make the supplement pointless. A 2023 meta-analysis of 27 studies found people with psoriasis have significantly lower vitamin D levels than healthy people (21.0 vs 27.3 ng/mL) and are three times more likely to be deficient. Chapter 7 recommends 5,000+ IU daily through the UVB-free months, and a home UVB lamp if possible.

A note on the target number. Chapter 7 gives an optimal range of 50–80 ng/mL; Chapter 18 gives 60–80. The book says it both ways and never reconciles them, so we are giving you both rather than picking. Either way it is a number to establish by testing, and the dose to reach it is a conversation with whoever ordered the test — vitamin D is fat-soluble and you can take too much.

One more thing: narrowband UVB phototherapy clears plaques in roughly 75% of patients and has done for decades. If you are having it, winter is the worst time to let appointments slide.

2. Your clock drifts, and the clock gates inflammation

This is the least intuitive mechanism and the best evidenced. Every skin cell carries clock genes — BMAL1, CLOCK, PER1, PER2, CRY1, CRY2 — running a 24-hour oscillation. In 2015, Japanese researchers found the CLOCK protein binds directly to the promoter of IL-23R, the receptor Th17 cells use to receive the IL-23 signal that drives psoriatic inflammation.

In a normally entrained person, IL-23R expression rises and falls daily: windows when immune cells are more responsive to inflammatory signals, and windows when they are less. Disrupt the rhythm and you lose the gating — instead of windows, constant IL-23 receptor expression. Two other findings agree: the circadian protein NFIL3 suppresses Th17 development by repressing RORγt, and mice lacking Bmal1 develop significantly worse psoriasis-like inflammation.

The human data comes from shift work. The Nurses' Health Study tracked over 100,000 women; those working rotating night shifts had a 19% increased risk of developing psoriasis.

Winter is not shift work, but it pulls the same lever more gently: you wake before it is light, commute in the dark, get less bright light by day and more artificial light at night. Chapter 7 also notes psoriasis patients have already lost their normal nocturnal melatonin peak — the rhythm is flattened before winter starts working on it.

3. Skin injury goes up, and injury is a trigger

Chapter 12 explains the Koebner phenomenon — psoriasis appearing at sites of skin injury — and it is the chapter that makes winter make sense.

Injure skin and you damage nerve endings, which release substance P and CGRP. Substance P binds MRGPRX2 on mast cells, which degranulate almost instantly — no antibodies, no allergen, just nerve signal in and inflammatory explosion out. Injured keratinocytes also release nerve growth factor, which makes nerve endings sprout, raises their substance P output and lowers the threshold at which they fire. The 10 to 20 day delay matches how long that sprouting takes.

Now list what winter adds: heavier fabrics, waistbands and collars, more friction at exactly the sites psoriasis already favours (elbows, knees, lower back, behind the ears — the areas Chapter 12 identifies as high mechanical stress with dense nerve supply). Drier skin. More scratching.

Scratching is the one to attack. Chapter 12 is blunt that it is not cosmetically bad but biologically destructive: every scratch triggers substance P release, mast cell degranulation and NGF production, and mast cell tryptase activates PAR-2 on nerve endings, lowering the threshold for the next itch. Each round rewires you to itch more easily — which is also why antihistamines barely touch psoriatic itch. The book's alternatives work on that mechanism: cold compress (cold inhibits nerve firing), gentle pressure (activates competing nerve fibres), distraction, and over-the-counter capsaicin cream (0.025–0.075%), which depletes local substance P over time and has a small double-blind trial behind it. The first applications burn; not everyone tolerates it.

4. Everything else winter changes

Stress. Chapter 12 cites animal work showing stress can raise substance P in skin-innervating neurons by 40–60% within days. The route from a stressful week to a flare is neural, not psychosomatic.

Movement. Muscle contraction releases myokines that raise IL-10 and IL-1ra while suppressing TNF-alpha, and exercise activates AMPK, which opposes the mTOR signalling driving psoriatic skin cell proliferation. Chapter 8 cites a 2024 study in which 20 weeks of activity with no dietary change halved severity in half of the 16 participants. Winter is when this stops happening.

Infection. Chapter 4 names streptococcal throat infection as a known psoriasis trigger. The book does not discuss seasonality, but strep is a winter organism, and a sore throat followed weeks later by a flare is worth raising with your doctor.

Alcohol. Winter carries a season of it, and the book's coverage is thin — five mentions in twenty chapters, no threshold, no distinction between drink types. If you take methotrexate this belongs with your prescriber: the book flags hepatotoxic medication twice and never once connects it to drinking.

What the book does not cover, and we are not going to invent

Heal Psoriasis never mentions moisturisers or emollients. Not once, in a book about a skin disease. It says nothing about indoor heating, humidity, shower temperature or bathing routine.

That is a gap, not a position — it does not mean the book thinks emollients are useless, it means the subject is absent. So this article is not going to tell you what to put on your skin in January and attribute it to a book that is silent on the question. If you use an emollient, or want to, ask a dermatologist or pharmacist, who can answer it properly for your skin. The silence is recorded on the corrections page with the book's other known gaps.

A winter plan built only from what the book supports

Protect the morning. Outside within 30 minutes of waking, light in your eyes, not through a window and not through sunglasses. Ten to thirty minutes, on the darkest mornings too — overcast outdoor light still beats indoor lighting by an order of magnitude.

Handle vitamin D deliberately. Test, then supplement to a number rather than guessing. Above 37° north, assume your skin makes none from November to February.

Protect the evening. Blue-blocking glasses two to three hours before bed if you use screens, dim warm lighting, no screens in the bedroom, blackout curtains. This is the half of the light protocol most people ignore, and the half that protects melatonin.

Keep the hormetic inputs. Do not let a sauna habit lapse — Chapter 8 offers a hot bath at 104°F for 20–30 minutes as a substitute. Finish showers cold for 30 seconds and build. Keep walking 30 minutes a day, because winter is the season that stops.

Reduce friction and stop scratching. Watch waistbands, collars and cuffs. Cold compress instead of nails.

Do not make it up in one weekend of spring sun. Chapter 7 asks for a "solar callus" built slowly — 5–10 minutes to start for very fair skin, longer for darker skin, aiming at mild pinkness and never burning. Sunburn causes DNA damage and inflammation that defeats the purpose, and Chapter 12 notes it is a Koebner trigger in its own right.

Take a photo and a score now. You will not remember January accurately in March, and a dated photo plus one repeatable severity number is the difference between knowing and guessing.

The honest limits

The book does not measure the winter effect. The seasonal pattern rests on Niels Finsen's 1903 observation plus Chapter 5 pointing "worse in winter, better in summer" at the light and hormesis roots. There is no effect size for winter in this book, and we will not invent one.
No trial has tested a winter protocol. Every recommendation above is a documented mechanism applied to a season. Light, circadian gating and Koebner each have real evidence individually; that they combine to produce your flare is a reasonable inference, not a finding.
Winter worsening is not universal. Some people flare in summer, from heat, sweat and friction. If that is you, your triggers are elsewhere.
None of this replaces treatment. Keep taking what you are prescribed. If winter is driving a serious deterioration, see your dermatologist sooner rather than trying harder alone.

Where to start

The free Can Your Skin Make Vitamin D Today? tool answers the question this whole article turns on, in about a minute: whether the sun where you are is currently strong enough to do anything at all. If it is not, you stop chasing it and supplement instead. The free Light & Circadian Plan Builder turns that into an hour-by-hour day built around your latitude, the season and your working pattern — and when the itch arrives at 11pm, the free Itch Breaker is two minutes of what to do instead of scratching.

Chapter 7 of **Heal Psoriasis** has the full light and circadian evidence; Chapter 12 has the nerve, itch and Koebner mechanism; Chapter 8 has the cold and heat protocols.

This article is educational information, not medical advice. It does not diagnose, treat, or replace care from a clinician who knows your history. Never start, stop, or change a prescribed medication based on anything you read here — talk to your doctor first. Individual results vary.