What causes SAD?

The exact causes of SAD are still unclear. However, there are several theories about what causes it and why some people are more vulnerable to it than others.

The effects of light

When light hits the back of the eye (the retina), messages are passed to the part of the brain that rules sleep, appetite, sex drive, temperature, mood and activity (the hypothalamus). If there is not enough light, regulation of these functions is likely to slow down and gradually stop. Some people seem to need a lot more light than others for their body to function normally, and are therefore more likely to develop SAD symptoms if there are low levels of light in the environment.

Low serotonin levels

There are several brain chemicals involved in SAD, but the main one is serotonin. People experiencing depression have been found to have lower levels of serotonin, particularly in winter. It is thought that the brain’s system for releasing and absorbing serotonin to regulate moods might not be working properly in people with SAD.

High melatonin levels

When it is dark, the pineal gland in the brain produces the hormone melatonin, which makes us sleep. When it becomes light again, the pineal gland stops producing melatonin, which makes us wake up. It has been found that people with SAD produce much higher melatonin levels in winter than other people. This is also what happens to animals when they hibernate.

However, the relationship between melatonin levels 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, most people diagnosed with SAD continue to experience the depressive symptoms of SAD. This suggests that melatonin is unlikely to be the only cause of SAD.

Disrupted body clock

Your brain sets your body clock (circadian rhythm) by the hours of daylight. One theory is that if you experience SAD, the part of the brain that sets your body clock does not work properly 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. Also, some experts have argued that it is natural for the body clock to try to slow down in the winter months.

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 a major loss or bereavement, an assault, or by serious illness. Also, it may have been triggered by a change in diet, medication, or use (or withdrawal from) street drugs and alcohol.


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