Quick answer

What is seasonal affective disorder (sad)?

Seasonal affective disorder (SAD) is a type of depression that follows a seasonal pattern — symptoms typically start in autumn, worsen through winter, and lift in spring. Low mood, low energy, oversleeping, carbohydrate cravings, and social withdrawal are common. Treatment includes light therapy, talking therapies such as CBT, antidepressants in more severe cases, and lifestyle measures including outdoor daylight exposure and exercise.

Seasonal affective disorder — when winter brings depression

Seasonal affective disorder (SAD) is a type of depression that follows a seasonal pattern. For most people in the UK, symptoms begin in autumn, peak in deep winter, and lift in spring as daylight increases.

SAD is more than winter tiredness — it is a recognised mental health condition that can significantly affect work, relationships, and quality of life.

Symptoms of winter-pattern SAD

  • Persistent low mood — most of the day, most days
  • Loss of interest in activities you usually enjoy
  • Fatigue and low energy — despite sleeping more
  • Oversleeping — difficulty getting up in the morning
  • Carbohydrate cravings and weight gain
  • Irritability and difficulty concentrating
  • Withdrawal from social contact
  • Feelings of hopelessness or guilt

Symptoms typically return at the same time each year — a key diagnostic clue.

Why does SAD happen?

Shorter days and less sunlight in autumn and winter are the main trigger. Reduced light may:

  • Increase melatonin — causing sleepiness
  • Reduce serotonin — linked to mood regulation
  • Disrupt circadian rhythm — body clock

SAD is more common in northern latitudes — including the UK — and in people with a family history of depression.

Diagnosis

A GP diagnoses SAD based on:

  • Depression symptoms meeting standard criteria
  • A seasonal pattern for at least 2 consecutive years
  • Winter episodes outnumbering non-seasonal episodes

Blood tests may check for thyroid disease or anaemia — conditions that mimic depression.

Treatment

Light therapy

A SAD lamp delivering 10,000 lux used for 30 minutes each morning — within an hour of waking — is effective for many people. Choose a lamp with UV filtering. Effects often appear within 1 to 2 weeks.

Talking therapies

Cognitive behavioural therapy (CBT) — available through NHS talking therapies (self-referral in England). Helps change negative thought patterns and build coping strategies.

Antidepressants

SSRIs such as sertraline or fluoxetine — particularly for moderate to severe SAD or when light therapy alone is insufficient.

Lifestyle measures

  • Get outside in daylight daily — even on cloudy days
  • Exercise regularly — boosts mood and energy
  • Keep a consistent sleep schedule
  • Eat a balanced diet — limit sugar crashes
  • Stay connected socially

Summer-pattern SAD

A minority experience SAD in spring and summer — with insomnia, poor appetite, weight loss, and agitation rather than oversleeping. Treatment still centres on talking therapies and antidepressants; light therapy is not the main approach.

When to seek urgent help

If you have thoughts of self-harm or suicide, call 999, go to A&E, or contact Samaritans on 116 123 (free, 24 hours). SAD is treatable — see depression for broader depression guidance.

Common questions

What is seasonal affective disorder?
SAD is a type of depression that comes and goes in a seasonal pattern. Most people with SAD have symptoms in autumn and winter that improve in spring. A smaller number experience summer-pattern SAD with low mood in warmer months. It is a recognised form of depression, not simply winter blues.
What are the symptoms of SAD?
Persistent low mood, loss of pleasure in activities, fatigue and low energy, sleeping more than usual, difficulty waking, craving carbohydrates and weight gain, irritability, difficulty concentrating, and avoiding social situations. Symptoms recur at a similar time each year.
What causes SAD?
Reduced exposure to sunlight in winter is the main trigger. Less light may disrupt the hypothalamus, affecting production of melatonin (sleep hormone) and serotonin (mood hormone). Circadian rhythm disruption and low vitamin D may also play a role. Genetics and living far from the equator increase risk.
Does light therapy work for SAD?
Yes — for many people. A SAD lamp producing 10,000 lux used for around 30 minutes each morning, ideally within an hour of waking, can improve symptoms within 1 to 2 weeks. Lamps must filter UV light. Light therapy is often combined with other treatments for best results.
What other treatments are available on the NHS?
Cognitive behavioural therapy (CBT) and counselling through NHS talking therapies (IAPT). Antidepressants — particularly SSRIs such as sertraline — for moderate to severe SAD. Lifestyle advice including regular exercise, outdoor daylight, and consistent sleep schedule.
How is SAD different from ordinary winter tiredness?
Winter tiredness is mild and does not significantly impair daily life. SAD is depression — symptoms are persistent, affect functioning, and follow a clear seasonal pattern year after year. If low mood lasts most of the day for 2 weeks or more, it needs assessment.
Can I prevent SAD returning each year?
Starting light therapy in early autumn before symptoms begin may prevent onset. Maintaining exercise, maximising daylight exposure, managing stress, and continuing prescribed treatments through winter all help. Discuss a prevention plan with your GP if SAD recurs annually.

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