What Causes SAD ?

Seasonal affective disorder is caused by a combination of low light levels coupled with slight differences in brain chemistry in individuals.

Light levels

It is thought that we each have a minimum daily requirement for light according to our sensitivity to it and our brain chemistry. Light intensity is measured in lux:-

• candlelight has an intensity of 1 lux
• light indoors is usually between 200 and 700 lux
• a sunny day outdoors at mid-day is about 100,000 lux!

As the minimal level of light intensity for beneficial effect in a SAD sufferer is around 2,500 lux, it is easy to see how we can get starved of light when we spend almost the whole day indoors.

Serotonin and Melatonin

From the latest research it appears that SAD is due to an abnormality in the balance of certain hormones in the brain of sufferers, particularly serotonin and melatonin.

When you have enough serotonin you feel calm, balanced and in control and a lack of it leads to depression and poor concentration.  Levels of serotonin are generally lower in the population as a whole in winter and people with SAD suffer this lack more than most. Desire for sweet and starchy foods may be the brain's response to needing to boost low levels of serotonin. As the effects are only temporary this can lead to almost constant cravings. For more information on the effects of serotonin, see Serotonin and SAD

Melatonin is the hormone which promotes sleep. If you don't have enough serotonin, melatonin can take over and make you feel sleepy or that you need an energy boost. Melatonin is only produced during the hours of darkness and therefore levels are lowest in spring and summer with their relatively shorter nights and highest in autumn and winter.

Dopamine is another brain chemical whose production is stimulated by light and suppressed by melatonin. Abnormalities in its production have also been linked to SAD.