Seasonal affective disorder (SAD) is a type of depression that has a seasonal pattern.
The episodes of depression tend to occur at the same time each year, usually during the winter.
The symptoms often begin in the autumn as the days start getting shorter. They’re most severe during December, January and February.
In most cases the symptoms of SAD begin to improve in the spring before eventually disappearing.
What causes SAD?
The exact cause of SAD isn’t fully understood, but it’s thought to be linked to reduced exposure to sunlight during the shorter days of the year.
Sunlight can affect some of the brain’s chemicals and hormones. However, it’s not clear what this effect is. One theory is that light stimulates a part of the brain called the hypothalamus, which controls mood, appetite and sleep. These things can affect how you feel.