The exact cause of seasonal affective disorder (SAD) is unknown.

The symptoms of depression are thought to be linked to the shorter days of the year during the winter months, when the body has less exposure to sunlight.

Experts across the world have been researching SAD since the beginning of the 1980s and have developed several theories about what may trigger the condition. These are discussed below.

Effect on the hypothalamus

The amount of sunlight you receive affects some of your body’s hormones and the chemicals in the brain. However, the effect that sunlight has on the brain isn’t fully understood.

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

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

• affect the production of the hormone melatonin
• affect the production of the hormone serotonin
• disturb your circadian rhythms (the body’s internal clock that regulates several biological processes)

Melatonin

Melatonin is a hormone that affects sleep. It’s produced by a small gland in your brain called the pineal gland. When it’s dark, the pineal gland produces melatonin to make you feel sleepy.

When it’s light, the retina (the light-sensitive layer of cells at the back of your eye) converts the light into an electric impulse that travels to the hypothalamus. The hypothalamus sends a message to several glands in the body, including the pineal gland, which produces less melatonin.

During the winter months some people produce higher than normal levels of melatonin, resulting in SAD symptoms such as sleepiness and low energy levels.

Serotonin

Serotonin is another hormone that affects mood, appetite and sleep. It’s also a neurotransmitter, which means that it transmits messages between nerve cells. When there are fewer hours of sunlight, less serotonin is produced.

If you have SAD, your serotonin levels may be lower than average during the winter. This may cause the messages between nerve cells to not be transmitted effectively, leading to the symptoms of SAD, such as feeling down.

Circadian rhythm

Your body uses light sources such as the sun to time various important biological functions and processes, including your mood, sleep, appetite, digestion and energy levels.

This regulation of the body’s functions is known as a circadian rhythm, which usually occurs over a 24-hour period. However, a lack of sunlight can sometimes disrupt the body’s internal clock, leading to SAD symptoms such as depression and low energy levels.

Other causes

Many different causes and contributing factors are associated with depression, some of which may also contribute to SAD. Possibilities include:

• genetic and family factors – a family history of depression could mean that you inherit particular genes that make depression more likely
• adverse childhood experiences – such as neglect or physical or sexual abuse
• personality – for example, you may be naturally quite anxious
• psychological factors – this relates to how the brain functions, such as your emotions and the way you think about things
• social factors – your relationships with other people, for example, whether or not you have close friends

Visit your GP if you have the symptoms of seasonal affective disorder (SAD). Effective treatments are available if you’re diagnosed with the condition.

Your GP may 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’s anything in your personal history that may contribute to a depressive disorder, such as child abuse
• whether there’s anything in your family history that may contribute to a depressive disorder, such as 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.

Diagnosing depression

Two major classification systems are used to diagnose depression. Your GP may use one of these systems to help diagnose SAD. The classification systems take into consideration:

• your symptoms
• the severity of your symptoms
• how long your symptoms last
• how your symptoms progress
• to what extent your symptoms prevent you from carrying out normal activities

You may be diagnosed with a level of depression that’s mild, moderate or severe. 

Seasonal depression

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

A diagnosis of SAD will depend on whether:

• you’ve had symptoms during the same seasons for two 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

The symptoms of seasonal affective disorder (SAD) are similar to those of normal depression, but they occur at a particular time each year.

Most people’s symptoms start in the autumn or winter and improve in the spring.

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

Some people with SAD experience recurring episodes of unipolar depression, while others may find they experience mild episodes of bipolar depression.

Unipolar depression is where you only experience depressive symptoms. Bipolar depression involves depressed periods (lows) followed by manic periods (highs) where you feel happy, energetic and much more sociable.

SAD symptoms

As with all forms of depression, the main symptoms of SAD include a low mood and a loss of pleasure or interest in normal everyday activities. Other depressive symptoms can include:

• feeling irritable
• feelings of despair
• feelings of guilt and worthlessness
• low self-esteem
• indecisiveness
• tearfulness
• feeling stressed or anxious
• a reduced sex drive

In addition to the above symptoms, you may also:

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

For many people, SAD can be difficult to live with and it can have a significant effect on day-to-day life. However, it can be successfully treated.

Read more about treating seasonal affective disorder. 

Seasonal affective disorder (SAD) can usually be effectively treated using various treatments, including cognitive behavioural therapy, antidepressants and light therapy.

Depending on the nature and severity of your symptoms, your GP will recommend the most suitable treatment option for you. This may involve using a combination of treatments to get the best results.

NICE recommendations

The National Institute for Health and Care Excellence (NICE) recommends that SAD should be treated in the same way as other types of depression.

This includes using talking treatments such as cognitive behavioural therapy (CBT) or medication such as antidepressants.

Light therapy is also a popular treatment for SAD. However, there’s no strong evidence to support the long-term benefits of using light therapy to treat the condition, although research does suggest that it may have a positive short-term effect.

See NICE guidance about the treatment and management of depression in adults.

Psychosocial treatments

Psychosocial treatments focus on both psychological aspects (how your brain functions) and social aspects (how you interact with others). Some possible psychosocial treatments are described below. 

Cognitive behavioural therapy

Cognitive behavioural therapy (CBT) starts with the idea that the way we think and behave may affect the way we feel. Changing the way you think about situations and what you do about them can help you feel better.

If you have CBT, you’ll have a number of sessions with a specially trained therapist, usually over several weeks or months. Your programme could be:

• an individual programme of self-help
• a programme designed for you and your partner (if your depression is affecting your relationship)
• a group programme that you complete with other people in a similar situation
• a computer-based CBT programme that’s tailored to your needs and supported by a trained therapist

Read more about CBT. 

Counselling and psychodynamic psychotherapy

Counselling is another type of talking therapy that involves talking to a trained counsellor about your worries and problems.

During psychodynamic psychotherapy you discuss how you feel about yourself and others and talk about experiences in your past.

The aim of the sessions is to find out whether anything in your past is affecting how you feel today.

Read more about psychotherapy.

Antidepressants

Antidepressants are often prescribed to treat depression and are also sometimes used to treat SAD when the symptoms are severe.

Selective serotonin reuptake inhibitors (SSRIs) are the preferred type of antidepressants for treating SAD. They increase the level of the hormone serotonin in your brain, which helps lift your mood.

If you’re prescribed antidepressants, you should be aware that:

• it can take between four and six weeks for the medication to take full effect 
• you should take the medication as prescribed and continue taking it even if you start to feel better
• some antidepressants have side effects and may interact with other types of medication you’re taking

An upset stomach is the most common side effect of antidepressants. See the information leaflet that comes with your medication for a full list of possible side effects.

There’s limited evidence to suggest that antidepressants are effective in treating SAD. They’re thought to be most effective if taken at the start of winter before symptoms appear, and continued until spring.

Read more about antidepressants.

Light therapy

For some people with SAD, using light therapy can help improve their mood considerably. It involves sitting in front of or beneath a light box.

Light boxes are special lamps that come in a variety of designs, including desk lamps and wall-mounted fixtures. They produce a very bright light. Light intensity is measured in lux – the higher lux, the brighter the light.

Before using a light box to treat SAD you should check the manufacturer’s information and instructions regarding:

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

Make sure that you choose a light box that is medically proven to treat SAD and produced by a fully certified manufacturer. The Seasonal Affective Disorder Association can provide you with a list of recommended manufacturers.

Very bright light may not be suitable if you:

• have an eye condition or eye damage that makes your eyes particularly sensitive to light
• have a condition that makes your skin particularly sensitive to light, such as systemic lupus erythematosus
• are taking medication that increases your sensitivity to light, such as certain antibiotics, anti-inflammatories, anti-psychotics or the herbal supplement St. John’s Wort
• have skin cancer

Speak to your GP if you’re unsure about the suitability of a particular product.

How light therapy works

Light therapy is thought to work by simulating the sunlight that’s missing during the darker winter months.

The additional light encourages your brain to reduce the production of melatonin (the hormone that makes you sleepy) and increase the production of serotonin (the hormone that affects your mood).

Altering the levels of melatonin and serotonin that are released into your body during the winter months can help ease your symptoms of SAD.

However, this is based on the assumption that the condition is caused by a lack of light and the effect that this has on the hormones that are released in your brain.

Research into light therapy

There’s mixed evidence regarding the overall effectiveness of light therapy. It’s difficult to compare the research that’s been carried out in this area because a variety of light sources were used and clinical trials (medical tests) were often conducted over short periods of time.

However, several studies have concluded that light therapy is effective, particularly if it’s used first thing in the morning. 

It’s thought that light therapy is best for producing short-term results. This may mean that it will help relieve your symptoms, but you’ll still be affected by SAD next winter.

Side effects of light therapy

It’s rare for people using light therapy to have side effects. However, in rare cases you may experience:

• mild agitation or irritability
• headaches
• sleeping problems (avoid using light therapy during the late evening)

Visit your GP if you experience any side effects while using light therapy.