Seasonal depression (SAD): the signs, the months and what light therapy can do

Winter blues or seasonal depression

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Plenty of people feel flatter when the days get short. They sleep a little longer, want to stay in more and look forward to spring. That is usually called the winter blues, and it does not stop ordinary life. Seasonal depression is the same direction taken much further: low mood and lost interest that settle in for weeks, with sleep, appetite and energy changing alongside, so that work, home and the people around you take a hit.

The National Institute of Mental Health describes it as a type of depression with a recurrent seasonal pattern, lasting about four to five months of the year. The NHS lists low mood, a lack of interest in things you usually enjoy, feeling irritable, difficulty concentrating, being more hungry and eating more than usual, and being more tired and sleeping more than usual among the signs.

One distinction matters more than it looks. NIMH says the depression follows changes in daylight, not the calendar. A December that is hard because of money, holidays with relatives or a work deadline is a different thing from a dark-months pattern that would arrive whatever the diary said.

Which months, and the summer pattern

For most people the pattern starts in late autumn or early winter and goes away during spring and summer. In the southern hemisphere that means the middle of the calendar year rather than the end.

A less common summer pattern also exists. NIMH describes it with trouble sleeping, poor appetite, restlessness and agitation rather than the long sleep and heavy eating of the winter pattern, and says less is known about it because it is harder to study.

The NHS also notes that some people feel more energetic than usual at the start of spring or summer. Usually that is simply the relief of lighter days. If the lift goes further, with much less sleep, racing plans or spending that is out of character, that is worth mentioning to a doctor, because seasonal changes are more common in people with bipolar disorder.

• Late autumn to early spring: the usual winter pattern.
• Late spring to late summer: the less common summer pattern.
• Two years or more of the same months: the history a diagnosis rests on.

What a clinician asks about

Both NIMH and the NHS describe the same starting point: a conversation about how your moods change over time. A GP or another clinician will want to know which months, for how long, what changed in sleep, appetite, energy and plans, and whether it eased when the season turned. To be diagnosed, NIMH says the seasonal episodes need to have happened for at least two consecutive years, and more often in that season than at other times.

You may be asked to fill in a questionnaire. Researchers and some clinics use published seasonal-pattern questionnaires, and the best known is the Seasonal Pattern Assessment Questionnaire (SPAQ). This page does not reproduce it or any part of it, and the self-check on this site is not based on it or adapted from it. A questionnaire informs that conversation; it does not settle it.

• Thyroid problems, low vitamin D and some medicines can produce the same tiredness and low mood, and a blood test can check what no questionnaire can.
• A job or a home calendar that peaks every winter, or grief that falls at one time of year, gives a seasonal shape without the light being the cause.
• Low mood that is not seasonal at all is common, and it needs help just as much.

Light therapy: what the evidence says

Light therapy means sitting near a bright light box for a set time each day, usually in the morning. NIMH lists it among the treatments for the winter pattern, typically at 10,000 lux for 30 to 45 minutes from autumn to spring, and the NHS says many people find it helps.

The evidence is thinner than the advice around it suggests. The NHS says light therapy is not usually available on the NHS because there is not enough evidence to tell whether it works. A Cochrane review that looked at whether light therapy prevents winter depression in people who have had it before found only one small trial, with evidence of very low quality, and could draw no conclusion.

If you have bipolar disorder, talk to a doctor before trying a light box. The same applies if you have an eye condition or take medicine that makes you sensitive to light, which the NHS and NIMH both mention.

Talking therapies and antidepressants are the other treatments both bodies describe. Alongside them, the NHS self-care advice is unglamorous and cheap: as much daylight as you can get, regular exercise, staying in touch with people, and the same bedtime and waking time every day.

When to get help

See a GP if you think you might have seasonal depression, if a low stretch lasts more than a couple of weeks, or if it gets in the way of work, home or the people around you. In the UK you can also refer yourself to an NHS talking therapies service.

The most useful thing to bring is dates. Write down the months it started and lifted in the last two years, what changed first, and two or three concrete examples: plans you cancelled, days of work that went, how long you slept.

The self-check on this site can help you find those words. It compares your darker months with the rest of the year across six patterns, gives no score and no verdict, and lets you tick the months yourself in a table that stays on your device.

And if you are not safe, the crisis numbers at the top of this page matter more than anything else on it.