Seasonal affective disorder (SAD) is a form of depression that people experience at a particular time of year or during a particular season. Most of us are affected by the change in seasons it is normal to feel more cheerful and energetic when the sun is shining and the days are longer, or to find that you eat more or sleep longer in winter. However, if you experience SAD, the change in seasons will have a much greater effect on your mood and energy levels, and lead to symptoms of depression that have a significant impact on your day-to-day life.
The cause of this 'phase delay' and why it has such a pronounced effect on mood and behaviour, is likely to be multifactorial, involving abnormalities at various levels along the retino-hypothalamic tract, its links with the pineal gland, and the metabolism of melatonin and serotonin. This suggests that genetic aberrations may underlie the various abnormalities, which cause SAD symptoms. Melatonin secretion occurs later in the night, and for longer periods during the early morning compared to healthy individuals.
This suggests that most patients with SAD become depressed in the winter, at least in part because of a phase delay in circadian rhythms relative to the sleep/wake cycle.according to world health organisation statistics should be 35-70% of people suffered with SAD.