Many of us admit to occasional bouts of "winter blues" when days grow shorter and temperatures begin their annual free fall. However, wintertime sadness and depression can be more persistent for those diagnosed with seasonal affective disorder (SAD).  

SAD is a psychological condition that causes people to experience recurrent bouts of depression during certain seasons of the year, particularly late fall and winter. In rare cases, SAD may strike patients during late spring and summer. Another uncommon condition causes SAD to strike in both summer and winter, with symptoms receding in the spring and fall.

Scientists remain unsure of exactly what causes SAD. However, research indicates a strong link between wintertime SAD and an absence of exposure to daylight. As the days get shorter in the late fall, the seasonal light change may alter a person's internal biological clock, known as circadian rhythm. This may in turn trigger depression. Increased production of the hormone melatonin also may be partly responsible.

People who live in northern climates know that shorter days and longer nights are a natural part of life from autumn's first yellowing leaves until the ides of March. As daylight decreases during the fall and winter, people may feel lethargic or irritable. Such episodes rarely require treatment and soon pass.

However, people with SAD usually experience more significant symptoms. In addition to daytime fatigue and lethargy, they may sleep for longer periods of time and have a strong desire for sugary or starchy foods. Such habits can quickly lead to weight gain, another symptom associated with SAD.