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Seasonal affective disorder (SAD)

Explains seasonal affective disorder, including possible causes and how you can access treatment and support. Includes tips for helping yourself, and guidance for friends and family.

What causes SAD?

The exact causes of SAD aren't clear – a range of different factors are likely to contribute and these can vary a lot between different people.

Research suggests that the causes are similar to causes of depression, but there are also factors which can lead specifically to SAD.

Possible causes of SAD include:

Effects of light

When light hits the back of your eye, messages go to the part of your brain that controls sleep, appetite, sex drive, temperature, mood and activity. If there isn't enough light, these functions can slow down and gradually stop.

Some people seem to need a lot more light than others. This may mean they're more likely to get SAD during the winter months.

“It's like someone has switched off the light suddenly. I'm plunged into darkness, which paralyses me and keeps me pinned down to my bed.”

Disrupted body clock

Your brain sets your body clock by the hours of daylight. One theory is that if you experience SAD in the winter, this part of your brain isn't working in the same way.

This could mean your body clock is out of sync with daylight, leading to tiredness and depression. Some researchers think this is because your sleep pattern starts at a different time.

Is SAD more common in certain countries?

SAD is thought to be more common in countries where there are greater changes in the weather and in daylight hours during different seasons. This includes England and Wales.

“The evening is endless. I would watch the clock and feel trapped in the dark.”

Problems with melatonin and serotonin levels

When it's dark, your brain produces a hormone called melatonin which helps your body get ready for sleep. The levels of serotonin, a brain chemical that affects your mood, are also affected by how much sunlight you get.

Some people with winter SAD seem to produce higher levels of melatonin and lower level of serotonin during winter. Research suggests these can contribute to winter SAD, but the exact relationship between them isn't clear.

“When winter comes and I feel the change in the seasons, I feel more drained. I find it very hard to motivate myself into getting dressed or out of bed.”

Weather and temperatures

We all have different experiences of particular seasons and types of weather. You might feel particularly uncomfortable in hotter or colder temperatures, which could contribute to you developing depression (or any existing depression worsening) at those times.

While more people are aware of SAD happening in winter, some people have more difficulty in warmer weather. Some studies have suggested that summer SAD is linked to higher temperatures and humidity. But more research is needed to understand why.

“Sunshine and heat make me feel defensive, misanthropic, angry, anxious, resentful and impatient. I don't want to see anyone, go anywhere or do anything. Even bright, low winter light depresses me. I feel under siege.”

Do stressful times of year cause SAD?

SAD is diagnosed if your depression has a seasonal pattern. Some of us might have other times of the year, we find especially difficult, such as Christmas or bereavement anniversaries when we feel depressed. Some other mental health problems can also have a seasonal pattern, like bipolar disorder.

These feelings aren't necessarily a form of SAD but they can still mean that you need extra support at certain times of year.

Our self-care tips for SAD have some suggestions for you to think about, and our pages on coping with loneliness and stress may also be helpful.

This information was published in April 2022. We will revise it in 2025.

References and bibliography available on request.

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