Seasonal affective disorder (SAD) is a form of depression that people experience at a particular time of the year or during a particular season. 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.

Most people feel more cheerful and energetic when the sun is shining and the days are longer and perhaps people tend to eat and sleep more in winter. For those experiencing SAD, the seasons have a greater effect on mood and energy levels, leading to symptoms of depression that can have a significant impact on day-to-day life.

For some people, SAD is so disabling that they cannot function in winter without continuous treatment. Others may experience a milder version called sub-syndromal SAD or ‘winter blues’.

Symptoms include sleep problems, lethargy, over-eating, anxiety, depression, apathy, social problems, loss of libido, mood changes and a weakened immune system.