It is common for people to speak of how "depressed" they are.  However, the occasional sadness everyone feels due to life's disappointments is very different from the serious illness caused by a brain disorder.  Depression profoundly impairs the ability to function in everyday situations by affecting moods, thoughts, behaviors, and physical well-being.
Depression strikes about 17 million American adults each year--more than cancer, AIDS, or coronary heart disease--according to the National Institute of Mental Health (…showed first 75 words of 512 total…

…showed last 75 words of 512 total…for sleep, an exaggerated belief in one's own ability, excessive talking, and impulsive and often dangerous behavior.

Symptoms of the disease include:  Being persistently sad or anxious.  Sleeping too much or too little, reduced appetite, and weight loss or increased appetite and weight gain.  Difficulty concentrating, remembering or making decisions, fatigue loss of energy.  Feeling guilty, hopeless or worthless, and thoughts of death or suicide are all symptoms of depression.