Seasonal affective disorder (SAD) is a type of depression that has a seasonal pattern.

The episodes of depression tend to occur at the same time each year, usually during the winter.

As with other types of depression, the two main symptoms of SAD are a low mood and a lack of interest in life. You may also be less active than normal and sleep more.

Read more about the symptoms of seasonal affective disorder. 

Winter depression

SAD is sometimes known as "winter depression" because the symptoms are more apparent and tend to be more severe at this time of the year.

The symptoms often begin in the autumn as the days start getting shorter. They're most severe during December, January and February.

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

What causes SAD?

The exact cause of SAD isn't fully understood, but it's thought to be linked to reduced exposure to sunlight during the shorter days of the year.

Sunlight can affect some of the brain's chemicals and hormones. However, it's not clear what this effect is. One theory is that light stimulates a part of the brain called the hypothalamus, which controls mood, appetite and sleep. These things can affect how you feel.

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

• production of the hormone melatonin
• production of the hormone serotonin
• body's circadian rhythm (its internal clock, which regulates several biological processes during a 24-hour period)

Read more about the effects of sunlight on melatonin, serotonin and the body's circadian rhythm.

Diagnosing SAD

You should visit your GP if you have the symptoms of SAD. They may carry out an assessment to check your mental health.

Your GP may ask you about your mood, lifestyle, eating habits and sleeping patterns, plus any seasonal changes in your thoughts and behaviour.

Read more about diagnosing seasonal affective disorder.

Treating SAD

As with any type of depression, SAD can be difficult to live with. It can make you feel tired, stressed and unhappy. However, it can usually be treated successfully.

Light therapy is often used to treat SAD. It involves sitting in front of or beneath a light box that produces a very bright light. Light boxes come in a variety of designs, including desk lamps and wall-mounted fixtures.

You should speak to your GP and read the manufacturer's instructions before using a light box to treat SAD.

Depending on the nature and severity of your symptoms, talking therapies such as cognitive behavioural therapy (CBT) or medication such as antidepressants may also be recommended.

Your GP will recommend the most suitable treatment programme for you, which may involve using a combination of treatments.

Read more about how seasonal affective disorder is treated.