Seasonal affective disorder (SAD)

Seasonal affective disorder (SAD) is a type of depression that has a seasonal pattern. The symptoms often begin in the autumn as the days start getting shorter. They are most severe during December, January and February. It’s sometimes known as ‘winter depression’.

In most cases, the symptoms of SAD begin to improve in the spring before disappearing.

Symptoms of SAD

The symptoms of seasonal affective disorder (SAD) differ throughout the year and from person to person.

The symptoms of SAD are usually fairly mild at the start of the autumn. They might get worse as the winter progresses and the amount of sunlight decreases. However, the nature and severity of the symptoms of SAD will vary from person to person.

If you have SAD, you may experience depressive symptoms like:

  • low mood
  • a loss of pleasure or interest in normal, everyday activities
  • irritability
  • feelings of despair
  • feelings of guilt and worthlessness
  • low self-esteem
  • indecisiveness
  • tearfulness
  • stress or anxiety
  • a reduced sex drive

You may also:

  • be less active than normal
  • feel tired and sleep more than normal (hypersomnia)
  • feel lethargic (lack energy)
  • find it difficult to concentrate
  • have an increased appetite and eat more than usual (hyperphagia)

For many people, SAD can be a difficult condition to live with because it can have a significant effect on your day-to-day life.

Some people with SAD experience recurring episodes of unipolar depression. This is where you only experience depressive symptoms. Others find they experience mild episodes of bipolar depression. This is when depressed periods are followed by manic periods where you feel happy, energetic and much more sociable.

What causes SAD?

The exact cause of seasonal affective disorder (SAD) is unknown. The symptoms of depression are thought to be linked to less exposure to sunlight.

One theory is that light stimulates a part of the brain called the hypothalamus. This part controls your mood, sleep and appetite, affecting how you feel.

In people with SAD, lack of sunlight and a problem with certain chemicals in the brain prevents the hypothalamus from working properly. The lack of light is thought to affect:

  • the production of melatonin – melatonin is a hormone that makes you sleepy; in people with SAD, the body may produce it in higher than normal levels
  • the production of serotonin – serotonin is a hormone that affects your mood, appetite and sleep; a lack of sunlight may lead to lower serotonin levels, which is linked to feelings of depression

your internal body clock (circadian rhythm) – your body uses sunlight to time important biological functions, like when you wake up, so lower light levels during winter may disrupt your body clock causing SAD

Other causes

There are many different causes and contributing factors associated with depression. Some of these may also contribute to SAD. Possibilities include:

  • genetic and family factors, like a family history of depression, could mean that you inherit particular genes (units of genetic material) that make depression more likely
  • childhood experiences, like neglect or sexual abuse
  • personality, for example if you’re someone who is naturally quite anxious
  • psychological factors, relating to how your brain interprets emotions and how you think about things

social factors, like your relationships with other people and whether you have close friends

Diagnosing SAD

SAD is often difficult to diagnose because the there are many other types of depression that have similar symptoms. It may take several years before you and your GP realise that your symptoms are forming a regular pattern.

Speak to your GP if:

  • you think you might have SAD

Your GP might carry out a psychological assessment to check your mental health. During the assessment, you may be asked questions about:

  • your mood
  • your lifestyle
  • your eating patterns
  • your sleeping patterns
  • any seasonal changes in your thoughts and behaviour
  • whether there is anything in your personal history that may contribute to a depressive disorder, like abuse
  • whether there is anything in your family history that may contribute to a depressive disorder, like a family history of depression

Your GP may also carry out a brief physical examination to check for any other possible causes of your symptoms.

A diagnosis of SAD will depend on whether:

  • you’ve had symptoms during the same seasons for 2 or more years in a row
  • your periods of depression are followed by periods without depression
  • there are any other obvious explanations or causes for your seasonal mood changes

Treating SAD

Your GP will recommend the most suitable treatment option for you. This may involve using a combination of treatments to get the best results.

Treatment options may include:

  • cognitive behavioural therapy (CBT) – a talking therapy. It helps you change how you think and react to situations that are making you unhappy
  • counselling – another type of talking therapy where you talk to a trained counsellor about your worries and problems
  • psychodynamic psychotherapy – where you can discuss how you feel about yourself, how you feel about others and experiences in your past

Other treatment options

If other treatments like CBT, counselling and psychotherapy aren’t helping, there are some other options that might be offered to you.

Antidepressants for SAD

Antidepressants are often only prescribed for SAD when the symptoms are severe. This is because there’s limited evidence that they’re effective.

It’s thought that they are most effective if they’re taken at the start of winter (before the symptoms appear) and continued until spring.

It can take between four and six weeks for antidepressants to take full effect. It’s also important to continue to take the medication as prescribed, even if you start to feel better.

Read more about antidepressants

Light therapy for SAD

Researchers think that light therapy works by simulating the sunlight that’s missing during the darker winter months.

This additional light encourages your brain to

  • reduce the production of melatonin (the hormone that makes you sleepy)
  • increase the production of serotonin (the hormone that affects your mood)

It’s thought that altering the levels of both hormones can help to ease your symptoms of SAD.

However, there’s mixed evidence about the effectiveness of light therapy. It’s difficult to compare the research because they use a variety of light sources. Also, many of the clinical trials (medical tests) were conducted over short periods of time.

The studies which showed that light therapy is effective discovered that it’s best if used first thing in the morning.

Light therapy produces short-term results. It may help to relieve your current symptoms, but you’ll still be affected by SAD next winter.

Trying light therapy

When buying a light box, make sure that you choose one that is:

  • made by a fully certified manufacturer
  • medically proven to treat SAD

Check the manufacturer’s information and instructions to see:

  • whether the product is suitable for treating SAD
  • the light intensity you should be using
  • how long you need to sit in front of the light

The SAD Association can provide a list of recommended manufacturers.

Very bright light may not be suitable if you:

  • have an eye problem or your eyes are particularly sensitive to light
  • are taking certain types of medication, such as antidepressants
  • have epilepsy, a condition that causes seizures (fits)

Speak to your GP if you’re unsure about a particular product or if you experience any side effects while using light therapy.


Last updated:
28 February 2024