The assumption is sometimes made -- and wrongly so -- that sadness and depression are the same thing. Others falsely conclude that depression is merely a more severe case of sadness. While there are similarities, there are also multiple differences between the two [sources: Koenig; NAMI]. Sadness is common to the entire population. Major depressive disorder affects a small, though significant, portion of society. Some researchers estimate that 5 to 8 percent of people deal with the condition, while others put the numbers as high as 12 percent for men and 20 percent for women [sources: Belmaker and Agam; NAMI].
Sadness comes as the direct result of a particular event. The effects of major depressive disorder can descend upon a person even when it would appear that everything is going well in his or her life [source: Payne]. A person dealing with the disorder will also experience physical changes -- and changes in mood. The physical changes may involve a lack of interest in activities which he or she would normally enjoy. Weight gain or weight loss and sleeplessness or excessive sleep is also common. He or she may become easily agitated or brought to tears by the smallest of events. Thoughts of suicide may be prevalent, and it's not uncommon for major depressive disorder to lead to sluggishness and slow movements [source: Belmaker and Agam]. The duration of the symptoms is one of the biggest indicators of major depressive disorder. They don't go away after a few hours or a few days -- the symptoms last more than two weeks at a time [source: Belmaker and Agam].
Major depressive disorder also goes by the names clinical depression or unipolar depression. Unlike bipolar depression, those who battle major depressive disorder don't tend to experience the high highs as well as the low lows. There isn't a manic component to their mood [source: NAMI].