To Sleep, or To Be Sad?

Okay, now there's medical research being done all the time, but this particular study really got me thinking.

New study offers evidence of a link between staying up late and risk of depression 

So how many teenagers have been diagnosed with depression? Seems like most of them. However, they're staying up late pretty much every day. Sometimes voluntarily, sometimes because life requires it of them. But we're also dismissing it as average teen behavior. I've read many, many times about how the average circadian rhythm for a teen involves late nights and sleeping in, so how does this all figure together? They naturally stay up late and sleep late, but those same habits are increasing the risk of depression? I don't quite know what to think about all that, but I do think it's worth some serious consideration.