Seasonal affective disorder (SAD): symptoms, causes, treatment and how long it lasts

The NHS lists low mood, a lack of interest in things you usually enjoy, feeling guilty, hopeless or worthless, restlessness or irritability, difficulty concentrating, eating and feeling hungrier than usual, being more tired and sleeping more than usual, and thoughts of suicide or self-harm. Some people also have a spell of feeling more energetic than usual at the start of spring or summer. NHS inform adds low self-esteem, indecisiveness, tearfulness, stress, anxiety and a reduced sex drive, and describes lethargy and craving or eating more as typical.

Not everyone has every symptom, and the pattern matters more than any single feature. NHS inform says that for many people SAD can have a significant effect on day-to-day life.

• Low mood that returns each winter.

• Less interest in usual activities and difficulty concentrating.

• Sleeping more and still feeling tired.

• Eating more than usual.

• Thoughts of suicide or self-harm need urgent help.

The NHS lists antidepressants, talking therapies such as cognitive behavioural therapy (CBT), and light therapy, and says many people need treatment every winter to stop symptoms returning. If you are 18 or over (16 or over in some areas) you can refer yourself directly to an NHS talking therapies service without seeing a GP.

Light therapy uses a bright light box or SAD lamp to simulate daylight. The NHS says it is not usually available on the NHS because there is not enough evidence to tell whether it works, but many people find it helps. NHS inform describes mixed evidence, says it may work better first thing in the morning, and that it produces short-term results, so you may still be affected the following winter. Both advise speaking to a GP first, because it may not be suitable if you have certain eye conditions or take medicines that increase sensitivity to light (NHS inform also lists epilepsy). If buying a light box, the NHS says to check it is bright enough (10,000 lux is standard), UV-free and carries a UKCA or CE mark.

NHS inform says antidepressants are often prescribed for SAD only when symptoms are severe, because evidence for them is limited. They may be most effective if started at the beginning of winter before symptoms appear and continued until spring, and can take 4 to 6 weeks to take full effect, so they should be continued as prescribed even if you feel better. Any decision about medicines belongs with your prescriber.

The NHS describes seasonal affective disorder as a type of depression that usually happens in the winter. NHS inform calls it a depression with a seasonal pattern, sometimes known as winter depression. Some people have symptoms in summer instead, but the NHS says this is less common.

Feeling a little low on dark days is common. SAD is different because it is depression, with symptoms that return in the same seasons year after year. NHS inform says that for many people SAD can have a significant effect on day-to-day life, and that it can be difficult to diagnose because many other types of depression have similar symptoms.

SAD follows the seasons, so it tends to come and go on a yearly cycle rather than lasting continuously. NHS inform says symptoms often begin in the autumn as days get shorter, are most severe during December, January and February, and in most cases begin to improve in spring before disappearing.

The NHS says the main symptom is depression that happens in the winter and goes away in the summer, and that many people need treatment every winter to stop symptoms coming back. NHS inform adds that light therapy produces short-term results: it may relieve current symptoms, but you may still be affected by SAD next winter.

Because the pattern repeats, some people find it helps to plan ahead. NHS inform says antidepressants, where they are used, are thought to work best if they are started at the beginning of winter before symptoms appear and continued until spring. Any plan like this should be agreed with your GP.

The exact cause is not known. Both NHS sources link it to reduced exposure to daylight in winter. The NHS says this may cause changes to brain chemicals such as melatonin and serotonin that affect mood and the body clock (circadian rhythm), and NHS inform describes one theory as a lack of light affecting mood, sleep and appetite. This is a theory rather than a proven mechanism, and it does not mean anyone has caused their own symptoms.

A GP will usually ask about your moods and how they change over time, and may ask you to fill in a questionnaire. The NHS says you may be diagnosed with SAD if you have had depression symptoms at a particular time of year for 2 years or longer.

NHS inform adds that a diagnosis depends on symptoms in the same seasons for two or more years in a row, periods of depression followed by periods without it, and no other reasons for the seasonal mood changes. It says your GP may ask about your mood, lifestyle, eating and sleeping patterns, seasonal changes in thoughts and behaviour, and your personal and family history, and may do a brief physical examination to check for other reasons for your symptoms.

SAD is often difficult to diagnose, because many other types of depression have similar symptoms. NHS inform says it may take several years before you and your GP realise that your symptoms form a regular pattern.

The NHS lists things that can help SAD symptoms. It suggests getting as much natural daylight as you can and trying to go outside in the daytime every day, taking regular exercise, staying in contact with friends and family, keeping up with your usual activities, and trying to go to bed and wake up at the same time every day.

It advises not drinking too much alcohol or using illegal drugs, and not using electronic devices such as smartphones right before going to bed. NHS inform points to a self-help guide for depression that uses cognitive behavioural therapy.

See a GP if you think you have SAD, or if you have been diagnosed and treatment is not helping.

The NHS says to see a GP if you think you might have SAD, or if you have been diagnosed and treatment is not helping. NHS inform advises contacting your GP practice if your mental health is affecting your daily life, you are not looking after yourself, or you are finding it hard to manage your responsibilities.

Thoughts of suicide or self-harm are listed by the NHS as a symptom of SAD and need urgent help. The NHS says support is available if you or someone you know is having a mental health crisis, no matter what you are going through. If someone is in immediate danger, call 999. You can also get help from NHS 111.

If you are under 18, the NHS says to look for mental health support for children and young people, and that people aged 18 or over (16 or over in some areas) can refer themselves to an NHS talking therapies service without a GP referral.

On the NHS, the route is a GP appointment or a self-referral to an NHS talking therapies service. NHS inform says your GP will recommend the most suitable treatment option and that this may involve a combination of treatments. The NHS also points to charities such as Mind and Rethink Mental Illness for information and support groups.

Light boxes are not usually provided on the NHS and many people buy them directly. NHS inform advises choosing one from a fully certified manufacturer that is medically proven to treat SAD, and checking with your GP if you are unsure about a product or have side effects. Private counselling and psychological therapy are other routes, and the same advice applies: check with a clinician that light therapy is suitable for you.