The exact causes of SAD are still unclear. However, there are several theories about what causes SAD and why some people experience more severe symptoms than others:
The effects of light
When light hits the back of the eye, messages are passed to the part of the brain that controls sleep, appetite, sex drive, temperature, mood and activity. If there’s not enough light, these functions are likely to slow down and gradually stop.
Some people seem to need a lot more light than others for their body to function normally. They are therefore more likely to develop SAD symptoms if there are low levels of light.
I like to think of Seasonal Affective Disorder as being solar powered – yeah it's pretty rubbish when winter comes around but it's nice to know things improve when the sun comes back.
Disrupted body clock
Your brain sets your body clock by the hours of daylight. One theory is that if you experience SAD, the part of the brain that does this isn't functioning completely and so your body clock slows down, leading to tiredness and depression.
However, as bright light (in the morning or from light treatment) appears to reduce symptoms of SAD, a problem with this part of the brain is unlikely to be the only cause of SAD.
Low serotonin levels
The brain uses the chemical serotonin to regulate our mood.
People experiencing depression have been found to have lower levels of serotonin, particularly in winter. It is thought there may be particularly strong seasonal variations in how this process works in people with SAD.
High melatonin levels
When it’s dark, the brain produces the hormone melatonin which makes us sleep. When it becomes light again, it stops producing melatonin and we wake up.
It has been found that people with SAD produce much higher levels of melatonin in winter than other people. (This is also what happens to animals when they hibernate).
The relationship between melatonin and SAD is still unclear. We know that if someone with high melatonin levels is exposed to bright light, their melatonin levels drop to normal. However, trials have shown that even after their melatonin levels have returned to normal, they still experience the depressive symptoms of SAD. This suggests that melatonin is unlikely to be the only cause of SAD.
Other possible triggers
Like other forms of depression, SAD has also been reported to have been triggered by:
- an unwelcome or traumatic life event, such as a major loss or bereavement, an assault, or by serious illness
- physical illness
- a change to diet or medication
- the use (or withdrawal from) street drugs and alcohol.
People who have lived near the equator for part of their lives and then moved to the UK seem to be particularly vulnerable to developing SAD.
This information was published in June 2016. We will revise it in 2019.