Depression caught early is eminently treatable and sleep problems go away. Even if they are not depressed, there are things that can be done about sleep. Either way, people need to get themselves evaluated. There are sleep specialists as well as psychologists, psychiatrists who can evaluate them for the presence of depression. This is an association that is well recognized.
Sleep disorders is one of the characteristics in the diagnostic measure of depression.
Monjan agrees that much insomnia is associated with physical problems, often independent of the person's age. The risk factors for depression and insomnia are very similar. He disagrees with Roberts' assertion that sleep problems is only a weak predictor of depression. I have a concern there. However, I think the paper's interesting from the point that it's another population-based study showing that sleep is an important predictor for depression.