Depression and insomnia - the chicken or the egg?

The relationship between depression and insomnia is complex - particularly for the patient. Most people suffering from poor sleep, even if only for a few nights, present with depressive-like symptoms but are generally not happy if they're told that poor sleep is merely a symptom of depression. Patients will often deny having depression even when there are clear symptoms and blame these on the lack of sleep. So is poor sleep always a symptom of depression?