What is it?
Seasonal Affective Disorder Syndrome or SADS (for short) is a form of depression. It usually occurs during months when the days are shortest, and the amount of sunlight (or just ordinary light) that we get is at its lowest. Therefore in the UK people tend to get it most in January and February.
What causes it?
It seems to be caused by a lack of light - especially sunlight. There is a bit of our brain called the "pineal gland" which is very sensitive to the amount of light which we get each day. This gland normally secretes a hormone at night called melatonin which helps us to sleep. However if we get too little light during the day, then our pineal gland goes into overdrive and produces tons of the stuff which makes us feel depressed.
Most of us probably feel a bit "down" during winter months anyway, and how much this is actually due to "SADS" is difficult to know.
Who gets it?
Some people seem to suffer more from it than others, but it's not clear exactly why - just part of the great human variation like being short or tall, maybe. However we do know that women are more likely than men to suffer from SADS but men do suffer from it as well.
Can it be treated?
A holiday in the sun or skiing makes most people feel better. But this is not something that most people can afford whenever they feel down. Otherwise you can buy a special light box at your local health store and sit opposite it for several hours a day. This can help you if you suffer badly from SADS. Most people can't find the time to do even this though, so only people who suffer very badly from it try this treatment.