TempoLife › Features › The dark season: light, sleep and vitamin D › mood
Winter blues and when it is depression
The difference between the seasonal dip and seasonal affective disorder, and the line where help is needed.
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.
The normal seasonal dip
Most people at this latitude notice the dark season: lower energy, more sleep, more appetite for starch, a flatter mood, less drive. It follows the light and lifts with it. That is the winter blues — a normal range of the seasonal variation, not a disorder, and it responds to the same levers as everything else on this page: morning light, movement, social contact, and not negotiating with the dark by sleeping through it.
When it becomes a diagnosis
Seasonal affective disorder is a real diagnosis — recurrent depressive episodes that arrive with the season and remit with it — and it is made by a clinician against diagnostic criteria, not by a questionnaire on a website. The pattern that separates it from the blues is severity and persistence: the low mood, the loss of interest in things that normally matter, the weight and sleep changes, the inability to function — week after week, season after season. The self-assessment pages score the standard questionnaires, and the result they give is a next step, not a diagnosis.
What helps, in the order of the evidence
Bright-light therapy has the most trial evidence for the seasonal pattern, used as dosed on the light therapy page. Morning exercise has solid evidence in depression generally and is doubly useful here because it happens in daylight. Cognitive-behavioural therapy has the strongest evidence of the talking therapies for the seasonal form specifically. Medication is a clinician's decision and a good one for many people. The order matters less than the fact that all four are real treatments, which is the answer to "is there anything that actually works".
The emergency line
If thoughts of self-harm or suicide are present, the step is not a light box: it is the crisis services now. The mental health support page carries the national crisis lines and the free routes into care for each country.
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
Is seasonal affective disorder real?
Yes — it is a recognised pattern of recurrent depressive episodes tied to the season, and it is diagnosed and treated by clinicians like other depressions. What is not real is the idea that everyone at this latitude has it; the winter blues are the normal end of the same spectrum.
When should I see a doctor about my winter mood?
When it stops being a dip: weeks of low mood and lost interest, sleep and appetite that will not settle, function that suffers, or any thought of self-harm. The diagnostic line is severity and persistence, and a clinician is the one who draws it.
The rest of the dark season
The dark season
All six winter pages, with the numbers behind them.
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.
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.
Track the season, not just the symptoms
TempoLife keeps sleep, steps and daylight in one picture, so the difference between your November and your June is on the chart instead of in your memory.