It is not clear whether sleep deprivation is a deciding factor in depression or if depression causes sleep deprivation. And the study was only done on one high school so there is not enough evidence to prove either theory. Also, another cause of sleep deprivation is high stress. This can come from too many activities plus homework, and keeping in contact with friends all of the time using cell phones, facebook, etc.
I think that it is possible that sleep deprivation and depression are not connected at all. There is not enough studying done on the issue to really know for sure if those two occurances have anything to do with each other.
Furthermore, it could be those specific students in that specific high school that seem sleepy and depressed. It may not be the trend of teenagers everywhere to be depressed and sleepy. The only way to find this out is if more studies are conducted in other high schools. Studies could also be done on adults too. There could be a connection between sleep deprivation and depression in them as well.

Then maybe high schoolers would sleep more and be less depressed. And we wouldn't have to conduct studies of their behavior to understand how to fix an outcome of stress and pressure.
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http://neuronarrative.wordpress.com/2009/06/12/when-its-learn-or-lose-sleep-on-it/