I have always functioned while depressed. Maintained a 4.0 in college despite depression. Have continuously worked full time as a teacher despite depression. I do have to say, I always could tell when I was beyond my usual chronic depression when I started having trouble functioning, trouble concentrating, more sleep issues, more anxiety. It has always been a matter of degree for me. I've spent most of my life functioning with a fairly constant degree of clinical depression, so much so that I simply learned to function through it because I didn't know there was anything different. But at times even that usual level of depression has increased to a much more severe, acute level that has caused me to become suicidal, require hospitalization, require meds and even ECT.