Seasonal Adaptation: What Should Change Through the Year

Vitamin D: The Largest Genuine Variation

At latitudes above roughly 37 degrees, cutaneous vitamin D synthesis is negligible for several months of the year because the sun does not rise high enough for adequate ultraviolet B to reach the surface. Status therefore peaks in late summer and troughs in late winter, and the swing can be substantial.

The practical consequence is that a fixed year-round dose is a compromise between two seasons. A dose adequate in August may be inadequate in February, and one appropriate for February may be unnecessary in August.

The reasonable approach: test at both ends, late summer and late winter, and set the winter dose from the winter reading. Daily moderate dosing rather than large intermittent doses, since some trials of large intermittent doses reported worse outcomes including increased falls.

Who needs more attention: people with darker skin, who synthesise less for a given exposure; people who cover their skin; those with rigorous sun avoidance, which is a Pillar 8 intervention creating a Pillar 7 problem; older adults, whose synthesis capacity declines; and anyone with malabsorption.

Interpretation note: comparing a February vitamin D result to an August one measures the season rather than a change, which is why testing at the same point each year matters for any other marker.

Light and Circadian Timing

Winter at higher latitude. Late dawn and early dusk reduce morning light availability, which delays circadian phase and makes waking harder. The intervention is light rather than sedation: getting outdoors early where possible, or using a light box, and maintaining a fixed wake time.

Seasonal affective disorder is a recognised condition with established light therapy evidence, typically 10,000 lux for 20 to 30 minutes in the morning. Subsyndromal winter low mood is common and responds to the same approach.

Summer at higher latitude. Early dawn can cause early waking, and warm nights impair the core temperature drop that sleep onset requires. Blackout and room temperature become the interventions.

Sleep duration tends to lengthen in winter and shorten in summer, which is a normal response to photoperiod rather than a problem to correct.

What not to do: use sedatives to address a winter timing problem, which addresses the wrong variable. The circadian protocol covers direction.

Activity and Training

The winter volume drop is the most consequential and least noticed change. Activity falls gradually, markers drift, and the drift gets attributed to something else. Tracking volume rather than relying on impression is the fix.

Heat acclimatisation is worth noting: it takes one to two weeks of consistent exposure, and performance in heat improves substantially over that period, which is why early-summer sessions feel disproportionately hard.

Skin, and the Vitamin D Tension

Sun protection does not change seasonally. Ultraviolet B varies dramatically with season while ultraviolet A does not, penetrates glass, continues on overcast days, and is the wavelength most associated with dermal matrix damage. Stopping sunscreen in winter is the most common seasonal skin error.

What does change: emollient richness rises in dry cold conditions, and active frequency may need to fall. Reducing retinoid frequency during a dry winter is reasonable, since it avoids the irritation that causes people to stop entirely, and cumulative benefit comes from consistency over years.

Indoor heating is the underappreciated winter factor. Months of low indoor humidity does more to skin than outdoor cold for most people, and a humidifier addresses the exposure directly.

The genuine tension. Rigorous sun protection reduces cutaneous vitamin D synthesis. This is a real trade-off between Pillar 8 and Pillar 7, and the resolution is supplementation rather than deliberate ultraviolet exposure, since the skin cancer and photoageing costs of the latter are established while the vitamin D benefit is achievable another way.

What Should Not Change

This half is as important as the adjustments, because seasonal thinking can license unnecessary churn.

Training structure. The four qualities, strength, power, bone loading and balance, are needed year-round. Only the modality changes.

Protein target. 1.2 to 1.6 g per kg for older adults, distributed, in every season.

Core supplements. Omega-3, creatine, magnesium and fibre have no seasonal rationale.

Sleep regularity. A fixed wake time matters more in winter, not less.

Sun protection, as above.

Monitoring cadence. Annual panels at the same time of year, quarterly functional tests, weekly behavioural review. Do not add tests because it is January.

Alcohol moderation, which holiday periods make harder rather than less necessary.

A protocol that changes wholesale every three months is not adapting, it is oscillating, and oscillation makes attribution impossible.

A Two-Point Annual Review

Rather than continuous seasonal fiddling, two scheduled reviews cover it.

Early autumn. Test vitamin D and set the winter dose. Plan indoor activity so volume does not drop by default. Switch to a richer emollient and consider reducing active frequency. Address indoor humidity. Book anything overdue, since winter is when appointments are easier.

Early spring. Test vitamin D again if you want the seasonal picture, and reduce the dose if appropriate. Move activity outdoors and rebuild volume gradually rather than immediately. Return to lighter skin formulations and normal active frequency. Raise sun protection diligence as the ultraviolet index rises, which happens earlier in the year than most people expect.

What to note at each: whether anything drifted, whether the core protocol is still being followed, and whether any component has been continuing without a current reason.

That is the whole seasonal protocol: two reviews, four variables, and everything else left alone. It is less interesting than a season-by-season regimen and considerably more likely to be sustained, which in a Pillar where adherence dominates outcomes is the relevant criterion.