The most common form of SAD is also called winter depression or winter blues, because symptoms are worst in the winter months. They tend to start from around September, are worse when the days are shortest (in December, January and February) and improve in the spring.
There is also a summer version of seasonal affective disorder, but this is far less common and has different symptoms. Around 1 in 50 people in the UK have SAD. It is more common in women than in men and most commonly starts between the ages of 18 and 40.
Up to 1 in 8 people in the UK experience milder symptoms of winter 'blues' (sub-syndromal SAD). Studies around the world have shown that SAD becomes more common the further you are away from the equator. This suggests that SAD is linked to the change in the number of daylight hours through the year.
The cause of SAD is not fully understood, but is thought to be linked to reduced exposure to light during the winter months. Researchers have shown that bright light makes a difference to brain chemistry, but it is not clear exactly what the effect is. One theory is that light stimulates a part of the brain called the hypothalamus, which controls mood, appetite, and sleep.
In people with SAD, lack of light and a problem with certain brain chemicals prevents the hypothalamus from working properly. Melatonin, a sleep-related hormone released in the brain, has also been linked to SAD. This hormone is thought to cause symptoms of depression and is produced at increased levels in the dark. SAD may also have a genetic cause. It has been shown that you are more likely to develop SAD if you have a close relative who has it.
It can sometimes be quite difficult to tell the difference between SAD and other types of depression. It may not be immediately obvious that the symptoms are linked to changes in the seasons, as people with non-seasonal depression can also have repeated cycles of symptoms worsening and improving, but for different reasons.
It is therefore important to try to spot the pattern of symptoms always appearing in the winter months, and disappearing in the summer on their own, not as a result of treatment. It usually takes at least two years to show this pattern clearly. A special questionnaire called the Seasonal Health Questionnaire is sometimes used to help specialists work out whether a person has SAD.