Maybe I should give a little introduction to depression. I'm talking full-on clinical depression here. That's what I have. Or I should say, have had, though I consider it an ongoing condition. I have had two major incidents of it but I'm always on alert.
According to Wikipedia, clinical depression affects 7-18% of people some time in their lives. Okay so Wikipedia isn't exactly the most reliable source on the planet as any idiot can edit it (and many do) but it's a good place to start.
It mentions Seasonal Affective Disorder (SAD) which is feeling down in the Winter time. I actually thought I had that once. When it lasted through Summer and into Autumn, there was a problem. Turns out, what I had was Depression with Melancholic Features.
Oddly enough, I didn't really feel sad as such. I felt totally empty though. Completely disconnected from life and the world. I felt like I wasn't really me. I wasn't sleeping and had a bunch of other symptoms that led to the diagnosis. It was the first time I was diagnosed, though I had it years before and only knew what it was when I finally got that diagnosis. So, according to Wikipedia, here are the main symptoms:
"Melancholia is characterized by a loss of pleasure (anhedonia) in most or all activities, a failure of reactivity to pleasurable stimuli, a quality of depressed mood more pronounced than that of grief or loss, a worsening of symptoms in the morning hours, early morning waking, psychomotor retardation, anorexia, or excessive guilt."
Try living with this for 6 months. Or a year. Or every year. That's what people with depression live with.
Some people get to such a stage that you can't miss it. But there are a lot of people out there living with it that you just won't know. They plaster on a smile and get on with their day, secretly dying inside. You just never know what's going on with people.