Life has its ups and downs.  Feeling unhappy or sad in response to disappointment, loss or frustration is a normal reaction.  However, clinical depression overwhelms and engulfs your day to day life, interfering with your ability to work, eat, sleep and have fun.  A person with severe depression has little to no interest in work or hobbies, and may even have trouble getting out of bed.  Learning how to understand its signs, symptoms and causes is the first step to overcoming the problem.