Study leader UCLA professor of psychiatry Dr. Michael Irwin and his colleagues proposed three hypotheses for this phenomenon- risk for depression would be higher among older people with a prior history of the disorder; among those with prior depression, sleep disturbance could predict a relapse or recurrence; and sleep disturbances could act as a risk factor for depression recurrence separate from other depressive symptoms.
And the study confirmed all three hypotheses.
"Insomnia is the most frequent sleep disturbance in depressed patients and is viewed as a symptom of current depression. But when sleep disturbances begin to emerge in an otherwise healthy adult who has experienced depression in the past, we found that it may serve as a precursor to another attack of depression," said Irwin.
The study included 351 adults, age 60 and older, out of which 145 had a prior history of major or non-major depression that was in full remission, while 206 had no prior history of depression or other mental illness.
The researchers examined the participants at four different times over a two-year period for depressive episodes, depressive symptoms, sleep quality and chronic medical disease.
Amongst the subjects with prior depression, 23 had a relapse, compared with only one person in the group without prior mental illness.
With the first group, researchers were able to predict depression recurrence based on individuals' sleep disturbance. According to Irwin, the association was established irrespective of other depressive symptoms, chronic medical disease or any use of antidepressants.
He said that the study is the first to demonstrate that sleep disturbances act as an independent risk factor for depression recurrence in older adults.
"Unfortunately, sleep difficulties are often considered to be a part of normal aging, and asking about and assessing the quality of an older person's sleep is frequently overlooked during routine doctor visits. The omission is particularly striking, since we know that sleep disturbance is associated with declines in health functioning and with increases in all causes of mortality in older adults," said Irwin.
He added: "And now, this study shows that sleep disturbance is often related to depressive disorders in late life, which carry further considerable risks for morbidity and mortality."
He further said that in order to identify older adults at risk for depression, a two-step strategy can be employed. One step involves assessment of whether individuals have had a prior episode of depression, the other whether they have current and ongoing sleep disturbance.
"Given that sleep disturbance is a modifiable risk factor, these findings tell us that we need to develop treatments that target sleep disturbances for the prevention of depression recurrence in older adults," he said.
The study will be published in an upcoming issue of the American Journal of Psychiatry and currently available online.