“Also, long-sleepers were more likely to report symptoms of depression,” Couldn't that also be interpreted as "People with depression are more likely to report long sleep durations?"

This is so difficult to verify as we rely on how many hours are reported as sleep. We all know that we can't really determine how much time we have slept or not. Also, what about the quality of sleep? I bet that shorter bouts of sleep that are higher quality are more healthy than longer bouts of sleep that are low quality.