At first, Maggie's parents thought she was slacking off. After the Thanksgiving break, she couldn't concentrate in class, and after school all she wanted to do was sleep. Her grades began to drop, and she rarely felt like socializing anymore. They were upset with her, but figured it was just a phase — especially because her energy finally seemed to return in the spring.
But when the same thing happened the next fall, and Maggie's mood and her grades plummeted again, they took her to the doctor, who diagnosed her with a type of depression called seasonal affective disorder (SAD).
About Seasonal Affective Disorder

A form of depression that follows a seasonal pattern, SAD appears and disappears at the same times each year. People with SAD usually have symptoms of depression as winter approaches and daylight hours become shorter. When spring returns and the days become longer again, they experience relief from the symptoms and a return to a normal mood and energy level.
Signs and Symptoms

Like other forms of depression, the symptoms of SAD can be mild, severe, or anywhere in between. Milder symptoms minimally interfere with someone's ability to participate in everyday activities, while more severe symptoms can interfere much more.

The symptoms of SAD are the same as those of depression, but occur during a specific time of year. It's the seasonal pattern of SAD — the fact that symptoms occur only for a few months each winter (for at least 2 years in a row) but not during other seasons — that distinguishes it from other forms of depression.

Symptoms of SAD may include:
Changes in mood: sadness, irritability, and/or feelings of hopelessness or worthlessness most of the time for at least 2 weeks; tendency to be more self-critical and more sensitive than usual to criticism; crying or getting upset more often or more easily
Lack of enjoyment: loss of interest in things that are normally enjoyable; feeling like tasks can't be accomplished as well as before; feelings of dissatisfaction or guilt
Low energy: unusual tiredness or unexplained fatigue
Changes in sleep: sleeping much more than usual (which can make it difficult for kids with SAD to get up and get ready for school in the morning)
Changes in eating: craving simple carbohydrates (i.e., comfort foods and sugary foods); tendency to overeat (which could result in weight gain during the winter months)
Difficulty concentrating: more trouble than usual completing assignments on time; lack of usual motivation (which can affect school performance and grades)
Less time socializing: spending less time with friends in social or extracurricular activities

The problems caused by SAD — such as lower-than-usual grades or less energy for socializing with friends — can affect self-esteem and leave people feeling disappointed, isolated, and lonely, especially if they don't realize what's causing the changes in energy, mood, and motivation.