Depression is an illness that involves the body, mood, and thoughts. In addition, depression affects how a person eats, how much he sleeps, what he feels about himself, and how he thinks about things. Depression is not the same as a passing blue mood. It is not a sign of personal weakness, or a condition wished away. People with depression cannot merely "pull themselves together" and get better. Without treatment, symptoms can last for weeks, months, or years. A mood disorder marked especially by sadness, inactivity, difficulty in thinking and concentration, a significant increase or decrease in appetite and time spent sleeping, feelings of worthlessness and hopelessness, and sometimes suicidal tendencies. characterizes the onset of Seasonal Affective Disorder during the winter months when there is less natural sunlight. This depression generally lifts during spring and summer. Winter depression, typically accompanied by social withdrawal, increased sleep, and weight gain, predictably returns every year in seasonal affective disorder.