As found on the Canadian Mental Health Association page.
Depression in the Workplace
One of Canada's most common illnesses is also the least understood. Everyone feels "blue" or sad from time to time. It's a normal life experience. But when these emotions increase in intensity, persist for more than a few weeks, and start to interfere with a person's life, it may signal depression. No amount of "cheering up" can make the depression go away; no amount of exercise, vitamins or vacation can make it disappear. That's because depression is an illness, not a weakness.
To read more visit: http://www.cmha.ca/mental_health/depression-in-the-workplace/