In this study almost a thousand older women (70 and older) were studied over the course of 5 years.   All the women at the start of the study were not depressed (according to measures designed to assess depression).  At the end of the study the results showed a connection that the women that struggled with lower quality sleep over the period of those 5 years had greater odds of experiencing worse depressive symptoms.

Take away:

This is one more piece of research that supports in a well established finding – poor sleep is not only associated with the occurrence or risk of depression but poor sleep can actually cause an episode of depression.  Related research has also indicated that poor sleep can cause relapses of depression episodes.