Nearly every tradition with a calendar organises the year into seasons with associated observances, dietary guidance and activity patterns.
Modern life has substantially removed seasonal variation — climate control, artificial light, year-round food availability. There's a research literature on whether that matters.
What varies seasonally in humans
More than commonly assumed.
Sleep. Studies have found seasonal variation in sleep duration and timing, with longer sleep and later timing in winter months, though the effect in modern indoor-living populations is smaller than in those with more natural light exposure.
Mood. Seasonal variation in mood is well documented, with seasonal affective disorder representing the clinical end of a spectrum that includes milder seasonal variation in a substantial proportion of people.
Vitamin D. Synthesis depends on UVB availability, which varies dramatically with latitude and season. Deficiency is markedly seasonal in higher-latitude populations.
Immune function. Research has found seasonal variation in gene expression related to immune and inflammatory function, with different profiles in winter and summer.
This finding was notable because it suggested seasonal variation is more pervasive at a molecular level than had been assumed.
Cardiovascular measures. Blood pressure shows seasonal variation, higher in winter, and cardiovascular events show seasonal patterning.
Birth timing. Conception and birth show seasonal patterns that vary by latitude and have changed over time.
Light as the primary signal
The mechanism underlying most of this appears to be light exposure.
Day length is detected through specialised cells in the retina, which signal to the circadian system. That system influences melatonin secretion, which varies with season in natural conditions.
Modern indoor living substantially compresses this signal. Indoor lighting is far dimmer than daylight, and artificial light at night extends apparent day length.
Which means many people receive a much flatter seasonal light signal than their physiology evolved with, and studies comparing people with more and less outdoor exposure find differences in seasonal variation.
Seasonal affective disorder
The clearest clinical manifestation.
Characterised by recurrent depressive episodes with seasonal pattern, most commonly winter onset, with symptoms including low mood, increased sleep, increased appetite particularly for carbohydrate, and fatigue.
Prevalence increases with latitude.
Light therapy — exposure to bright light, typically in the morning, using a device of specified intensity — has reasonable evidence and is a first-line treatment.
Cognitive behavioural therapy adapted for seasonal presentation also has evidence, and some studies suggest better long-term outcomes than light therapy alone.
Milder seasonal variation, sometimes called subsyndromal, is considerably more common than the full disorder and may respond to similar approaches.
What traditions prescribed
Worth looking at, since the guidance is remarkably consistent.
Winter: more rest, more sleep, warming food, reduced activity, indoor and communal life. Multiple traditions specify this.
Spring: cleansing practices, dietary change, increased activity. Fasting observances cluster here in several traditions.
Summer: activity, lighter food, more time outdoors.
Autumn: preparation, preservation, gathering.
Some of this is straightforwardly practical — food availability determined diet, and agricultural work determined activity.
The rest is harder to assess. Whether the prescription to rest more in winter reflects an accurate observation about physiological need, or simply what was possible, is not recoverable.
The modern flattening
What's changed.
Indoor temperature is constant. Food availability is constant. Working hours don't vary. Artificial light extends the day year-round. Activity patterns are the same in January and July.
Whether that flattening has consequences is not well established. The physiological variation persists, and the environment no longer varies with it, which is at least a mismatch.
Some researchers have proposed that the mismatch contributes to sleep and mood problems. The evidence is suggestive rather than demonstrated.
Practical suggestions
Things with reasonable support.
Get outdoor light, particularly in the morning, particularly in winter. Outdoor light even on an overcast day is far brighter than indoor lighting, and morning exposure supports circadian timing.
This is the single most useful intervention and it's free.
Consider vitamin D supplementation in winter at higher latitudes, consistent with public health guidance in many countries.
Allow more sleep in winter if your schedule permits. The physiological tendency towards longer sleep in darker months is real, and fighting it produces accumulated debt.
Reduce evening light in winter rather than compensating for short days with bright evenings, which pushes circadian timing later.
Seek help for significant seasonal mood change rather than treating it as inevitable. It's treatable and frequently untreated.
And there's something to be said for the traditional guidance to accept the season rather than to maintain identical output year-round. Whether or not the physiology requires it, the expectation of constant productivity regardless of daylight is a recent one, and not obviously an improvement.
General information only. Persistent low mood should be discussed with a qualified healthcare professional.
Latitude matters enormously
A point that gets lost when this material circulates internationally. The magnitude of seasonal variation depends heavily on where you are.
Near the equator, day length varies by roughly an hour across the year. At high latitudes it varies by many hours, with periods of near-continuous daylight and near-continuous darkness.
Which means advice about seasonal adjustment developed in one place transfers poorly to another, and prevalence of seasonal mood difficulty varies correspondingly.
Traditional seasonal guidance is similarly local, developed for particular latitudes with particular agricultural cycles. Applying a framework developed for one climate in another produces recommendations that do not correspond to anything happening outside.