Here is some of what the article said:
"dolescents with a late bedtime (midnight or later) were 24 percent more likely to suffer from depression. Adolescents who reported sleeping less than 5 hours per night were 71 percent more likely to suffer from depression"
So what might be some of the causes for this? Well during sleep many different chemical processes are happening. Recently I read about how proteins called Sleep Regulatory Substances are produced by neurons in the brain and may be one of the causes for us to sleep. If we didn't sleep and take time to get rid of these proteins then we could feel groggy all day and of course be depressed. I tend to fall on the slightly older explanation that serotonin production is the cause of this depressive behavior.
Though recently an article was released where depression is thought to be no longer caused by the depletion of serotonin in the brain by because of a lowered amount of dopamine in the brain. Serotonin levels have something to do with this but its not clear how yet. At any rate, during sleep serotonin is one of the many chemical processes occurring, and a loss of sleep would be a huge damper on the amount of serotonin in the brain. The result, a sad face for the rest of the day.
You can read more about the article here: