The exact causes of SAD aren't clear. It seems that the things we know can cause depression in general can lead to SAD, and research has also suggested that there are a few things that could contribute to the development of SAD in particular.
Depression can vary a lot between different people and you might have SAD due to a combination of factors, or there might not seem to be any specific reason.
On this page you can find information about possible causes of SAD:
The 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 winter months. Some people seem to have the opposite experience, finding bright light and sunshine hard to cope with.
It’s like someone has switched off the light suddenly and I am 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, this part of your brain isn't working in the same way. This could mean your body clock slows down, leading to tiredness and depression.
Some researchers think this is because your sleep pattern (also known as your 'sleep phase') starts at a different time. This is sometimes described as having a delayed sleep phase.
Daylight and SAD
SAD is thought to be more common in countries where there are greater changes in the weather and daylight hours during different seasons, including England and Wales. People who live near the equator for part of their lives and then move further away may also be especially vulnerable to getting SAD.
The evening is endless and I would watch the clock and feel trapped in the dark.
High melatonin levels
When it's dark, your brain produces a hormone called melatonin which helps your body get ready for sleep. Some people with SAD seem to produce much higher levels of melatonin during winter (which is also what happens to animals when they hibernate).
The exact relationship between melatonin and SAD isn't clear. Researchers have found that if you have high levels of melatonin and you're exposed to bright light, your melatonin levels drop to a more usual amount. But this doesn't seem to help with symptoms of depression.
When winter comes and I feel the change in the seasons, I feel more drained and 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 a possible link between higher temperatures and poor mental health, but more research is needed to understand why.
Sunshine and heat make me feel defensive, misanthropic, angry, anxious, resentful, impatient and turn my thoughts inwards. I don’t want to see anyone, go anywhere or do anything. Even bright, low winter light depresses me. I feel under siege.
This information was published in February 2019. We will revise it in 2022.