Seasonal affective disorder (SAD) is a type of depression that sets in or starts in the winter months. Unlike other types of depression, it may improve as spring comes on. It is often a cyclical, recurring disorder--you'll feel depression every winter and begin to feel better each spring.
A study by Alfred Lewy at the Oregon Health and Science University found that most of the symptoms of seasonal affective disorder are caused by body rhythms that are not in sync with the sun. Often the body gets out of sync with the sun because of winter's shortened days. The pineal gland, which regulates body rhythms, secretes a hormone called melatonin during the night. This can affect your sleep wake cycle and other rhythms. When it gets dark too early in the day, the pineal gland and the body's sleep wake cycle get out of sync.
Seasonal affective disorder is far more common in northern climates, where days can be very short in winter. SAD affects more women than men and is likelier to occur in people under age 40 than those older than that.