What you need to know about seasonal affective disorder (SAD)

What is seasonal affective disorder (SAD)?

SAD is a form of depression linked to the change in seasons. Symptoms often begin in the autumn as daylight decreases and typically improve during the sunnier months. While most people experience winter SAD, a small number may experience a summer pattern where symptoms appear in warmer months. 

Like other forms of depression, SAD can affect how you feel, think, and behave. You might find your mood low most days, lose interest in activities you usually enjoy, and struggle with energy or sleep. It’s not just a “winter slump” – SAD is a recognised mental health condition that can have a serious impact on daily life if untreated. 

What are the symptoms of SAD?

The symptoms of seasonal affective disorder can vary in severity from mild to severe. They tend to build gradually as the season progresses. The most common symptoms include: 

• Persistent low mood or sadness 

• Loss of interest or pleasure in normal activities 

• Low energy and constant tiredness 

• Sleeping longer than usual but still feeling fatigued 

• Difficulty concentrating or making decisions 

• Changes in appetite – often craving carbohydrates or gaining weight 

• Feeling irritable or tearful 

• Withdrawal from social contact 

• Feelings of hopelessness, guilt, or worthlessness 

Some people also experience physical symptoms such as slowed movement or speech, and increased sensitivity to rejection. If these symptoms persist for most of the day, nearly every day, it’s important to seek medical advice.

What causes seasonal affective disorder?

The exact cause of SAD isn’t clear, but it’s believed to be linked to reduced exposure to sunlight during shorter autumn and winter days. Several factors may contribute: 

• Disrupted body clock (circadian rhythm): Lower daylight can affect the body’s natural sleep–wake cycle, leading to tiredness and mood changes. 

• Reduced serotonin levels: Sunlight influences the production of serotonin, a brain chemical that affects mood. Low levels are linked to depression. 

• Increased melatonin: Darkness triggers melatonin production, a hormone that regulates sleep. Excess melatonin can cause lethargy and drowsiness. 

• Genetics: SAD may run in families, and you’re more likely to experience it if you have a relative with depression or bipolar disorder. 

• Vitamin D deficiency: Lower sunlight exposure can reduce vitamin D levels, which may influence serotonin activity and mood. 

How is SAD diagnosed?

If you think you might have SAD, speak to your GP. They’ll ask about your symptoms, how long you’ve had them, and whether they follow a seasonal pattern. Diagnosis is based on your history rather than a specific test, but your GP may carry out physical checks or blood tests to rule out conditions with similar symptoms, such as thyroid problems or anaemia. 

You may also be asked to complete a questionnaire to assess the severity of your depression. SAD is usually diagnosed when symptoms occur during the same seasons for at least two consecutive years and significantly interfere with your daily life or wellbeing. 

What seasonal affective disorder treatment options are available?

Treatment for SAD depends on how severe your symptoms are. For some people, simple lifestyle adjustments can help; for others, a combination of therapy, medication, and light exposure may be needed. Treatment follows recommendations from the NHS and NICE. 

1. Light therapy (SAD lamps) 

Light therapy is often the first treatment option for mild to moderate SAD. It involves sitting near a specially designed light box or lamp (not currently available on the NHS) that mimics natural daylight, usually for around 30 minutes to an hour each day. The bright light can help regulate melatonin and serotonin, improving mood and energy levels. Evidence for light therapy is mixed, although some studies suggest it may reduce symptoms in certain individuals. 

If you’re considering one, look for SAD lamps available in the UK that provide at least 10,000 lux and meet UK safety standards. Dawn simulators, which gradually brighten in the morning to mimic sunrise, may also help with waking and energy levels. 

Light therapy isn’t suitable for everyone, particularly people with eye conditions or certain skin sensitivities, so check with your GP before starting. 

2. Lifestyle changes 

Making positive daily changes can support your treatment and overall wellbeing. Helpful adjustments include: 

• Spending more time outdoors during daylight hours 

• Exercising regularly, even a short daily walk 

• Eating a balanced diet rich in fruit, vegetables, whole grains, and oily fish 

• Limiting alcohol and caffeine 

• Keeping a consistent sleep routine 

• Taking vitamin D supplements during autumn and winter to supplement levels during winter months, as generally recommended for your health by the NHS 

Staying connected with others, planning enjoyable activities, and prioritising relaxation can also help lift your mood. 

3. Cognitive behavioural therapy (CBT) for seasonal depression 

CBT is a structured form of talking therapy that helps you identify and challenge unhelpful thoughts and behaviours. It’s effective for many types of depression, including SAD. 

CBT can be delivered in person, online, or through guided self-help programmes. It can help people reframe negative thought patterns linked to winter and develop coping strategies for low mood and fatigue. 

4. Medication 

If your symptoms are moderate to severe, your GP may prescribe antidepressants, usually selective serotonin reuptake inhibitors (SSRIs) such as sertraline or fluoxetine. These help increase serotonin levels in the brain and can be particularly helpful when used alongside CBT or light therapy. 

Your GP will discuss the benefits, possible side effects, and how long you may need to take them. It can take several weeks for antidepressants to take effect, so patience is important.

Can SAD be prevented?

While you can’t always prevent SAD entirely, you can take proactive steps to manage symptoms before they worsen: 

• Begin light therapy in early autumn if you’re prone to SAD 

• Keep regular sleep and meal routines 

• Stay physically active and spend time outdoors 

• Plan social activities and enjoyable hobbies during winter months 

• Monitor early warning signs, such as low energy or disrupted sleep 

Building a self-care plan can make seasonal transitions easier to manage.

When to seek help

Everyone experiences changes in mood during the darker months, but if you feel persistently low for more than two weeks or find it difficult to carry out daily tasks, contact your GP. 

If you ever feel unable to cope, or experience thoughts of self-harm or suicide, you should seek help immediately by calling NHS 111, contacting your local urgent mental health helpline, or reaching out to Samaritans on 116 123 for free, 24-hour confidential support. 

Living well with SAD

Seasonal affective disorder can be challenging, but it’s important to remember that effective help is available. Combining self-care, professional guidance, and, where appropriate, light therapy or medication, can make a real difference to your quality of life. 

By recognising the signs early and seeking help, you can manage symptoms, maintain balance through the seasons, and feel more in control of your mental wellbeing year-round.