It might not be chemical for everyone, in my case it is...
And the SAD of course...

Some people don't believe in chemical imbalances.. but too me it makes perfect scense when you talk in terms of seretonin levels in your brain dipping past that invisble line that can cause depression... I beleive it can be.

But others don't think so.... it all has to do with serentonin.
Unless of course if the depression is caused otherwise, greiving, etc.