TempoLife › Features › The dark season: light, sleep and vitamin D
The dark season, in numbers
What the Nordic dark season does to sleep, mood and vitamin D, and the six answers that have numbers behind them: the 10 000-lux light-therapy dose, the winter supplement guidance, the daylight arithmetic, and where the line between the blues and depression actually sits.
This is seasonal self-care information, not a diagnosis and not treatment advice. Seasonal affective disorder is a clinical diagnosis made by a clinician; bright-light therapy is not safe to self-start for everyone (bipolar disorder and some eye conditions come first); and any measured vitamin D deficiency is treated at a clinician's dose, not a supplement-shelf one. If thoughts of self-harm are present, the step is crisis services now — the <a href="/therapists">mental health page</a> carries the national crisis lines.
How to read this
The six pages below are the six questions a northern winter actually raises, in the order people hit them: the light dose, the vitamin, the sleep, the mood, the daylight arithmetic, and the lamp purchase. Each page is built around the published numbers rather than the seasonal advice that circulates without them — the lux figures, the microgram recommendations, the trial doses — and each one says where the numbers stop and a clinician starts.
Light therapy: the dose that works
10 000 lux for 30 minutes in the morning — the dose the trials actually used, and how to deliver it.
Vitamin D in the dark season
What latitude does to skin synthesis, and what the Nordic recommendations actually say about supplements.
Winter sleep: longer, later and how to steer it
The dark season pulls sleep later and longer; the lever that fixes it is morning light, not evening discipline.
Winter blues and when it is depression
The difference between the seasonal dip and seasonal affective disorder, and the line where help is needed.
The daylight arithmetic: how little light we actually get
Indoors is a cave even at noon in summer; outdoors on a winter day beats any lamp. The lux numbers explain why.
Choosing a light box, wake-up light or daylight lamp
The three lamp categories, what each one is actually for, and the numbers that separate them.
The two facts the season is built on
First: light is the clock. The human circadian system runs slightly long on its own and is corrected every morning by bright light in the first hours after waking — and in winter, at this latitude, that correction is weak because the sun is low and indoor light is a cave. The drift that follows is why winter sleep runs late and why the fix is morning light, not evening discipline.
Second: the same low sun is why the skin stops making vitamin D. UVB is filtered out when the sun is low, from about October to March here, which is why the region's national recommendations converge on a winter supplement. Those two facts — the light and the UVB — are the whole of the dark season; everything else is downstream.
Light therapy dosing: the bright-light trials (10 000 lux, 30 minutes, morning). Vitamin D: the Nordic national nutrition recommendations. Circadian facts: the standard chronobiology literature (melanopic response, dim-light melatonin onset). The depression line: clinical diagnostic practice — seasonal affective disorder is diagnosed by a clinician. Checked 2026-09-06.
Frequently asked questions
Why is winter different at this latitude, biologically?
Two things go missing together: the morning light signal and the UVB. The low sun means the circadian clock loses its strongest anchor — the bright light that should hit the eye shortly after waking — so sleep drifts later and melatonin lingers into the morning. And the same low sun means no vitamin D synthesis from about October to March. Everything else on this page follows from those two facts.
Does a light box actually work?
For the seasonal pattern, the trials say yes, modestly: about 10 000 lux for 30 minutes in the morning, with most of the benefit felt within a week or two. The dose and the timing are the treatment — a box used at arm's length for half an hour in the morning is therapy, the same box used across the room in the evening is a lamp.
Should everyone at this latitude take vitamin D in winter?
The national recommendations in the region advise 10 µg (400 IU) a day for adults and children and 20 µg (800 IU) for the over-75s, with a supplement through the winter for people who get little sun and little fortified food — which is most people. A measured deficiency is a clinician's problem with a clinician's dose.
How do I tell the winter blues from depression?
By severity and persistence, and the honest answer is that a clinician draws the line, not a questionnaire. The blues follow the light, lift with it and respond to morning light, movement and contact. Depression that needs help is weeks of low mood and lost interest, sleep and appetite that will not settle, and function that suffers — season after season.
What is the cheapest light therapy there is?
Going outside. An overcast winter day is still thousands of lux — ten to a hundred times any indoor lighting — and the eye responds to it. Half an hour outdoors on a dull December noon delivers more circadian light than a day indoors under electric light, and it costs nothing.
The season you actually lived
TempoLife tracks sleep, steps and daylight across the year, so a dark November and a bright June are two different pictures on the same chart.