NEW YORK (Reuters Health) - People who struggle to get a good night's sleep are twice as likely, on average, to suffer from depression down the road as those who sleep soundly, Japanese researchers report.

After surveying thousands of adults in a rural Japanese town over the course of two years, a team from Tokyo Medical University found that participants with insomnia at the study's outset were up to two and a half times more likely to have a new or ongoing case of depression by the end.

"The study is interesting because it confirms again that there is a connection between sleep abnormalities and depression," said Dr. Srinivasan Pillay, an assistant clinical professor of psychiatry at Harvard Medical School, who was not involved in the work.

"What it does not tell us," Pillay told Reuters Health, "is whether sleep abnormalities are the cause of depression or something else. It tells us there is an association between the two."

Because a growing body of research shows a link between insomnia and depression, researchers led by Dr. Yuichi Inoue in the Department of Somnology at Tokyo Medical University set out to see whether it held true over time in a Japanese population. They also looked at whether specific types of insomnia were especially tied to future depression risk.

It was already known that one-fifth of the Japanese population reportedly suffers from insomnia and that sleep problems were a suspected risk factor for other psychiatric disorders such as anxiety, depression and substance abuse. But little research had been done in Japan.

Starting in 2005, Inoue's team administered two surveys to nearly 3,000 adults in the town of Daisen in Tottori Prefecture. One survey was designed to measure insomnia symptoms and the other to detect depression.

The insomnia survey let researchers analyze categories of sleep disturbance, including sleep quality, the amount of time participants took to get to sleep, sleep duration, sleep "efficiency" and daytime dysfunction.

In 2007, a little over half of the original participants agreed to take the two surveys again.

Inoue's group found that quality of sleep was more important than quantity in predicting a person's likelihood of depression and its severity.

Poor sleep quality and daytime dysfunction raised depression risk by 60 percent and 80 percent, respectively, compared to people who didn't have sleep problems or depression at the outset. Disturbed sleep raised depression risk by 30 percent and difficulty falling asleep or use of sleep medication each upped risk by 20 percent.

The researchers also looked at depression's influence on sleep and found that a new case of depression raised the risk of having insomnia by 7-fold and ongoing depression raised it by 3-fold.

Inoue could not be reached for comment, but his team concludes in the Journal of Clinical Psychiatry that insomnia should be seen as a red flag for increased risk of depression and should be treated "cautiously" to prevent depression from setting in.

Dr. Carole Lieberman, a psychiatrist on the clinical faculty of UCLA's Neuropsychiatric Institute said she found flaws with the report.

"This study does not prove that insomnia causes depression, but rather that insomnia is often associated with depression. We know that already," she told Reuters Health. "One of the classic symptoms of depression is sleeping too much or sleeping too little. In fact, early morning awakening has long been considered a...particularly characteristic symptom of depression. This study mentions this, but does not seem to have specifically tested for it."

She also noted the study's finding that many of the people who reported insomnia in their first survey were already depressed.

"The fact that they had insomnia and/or depression two years later is not surprising and is more reflective of their not being treated for their underlying depression," Lieberman said.

One lesson the study offers is the importance of getting treatment when insomnia sets in, said Daniel Taylor, a sleep researcher and associate professor at the University of North Texas.

"If you are having problems with sleep, get assessed by a sleep doctor, a board certified sleep specialist because the research is very consistent that you are going to be at risk for developing depression down the line," Taylor said. "If it turns out that you have insomnia, you should probably get treatment by a behavioral sleep specialist because it looks like sleep medication continues to put you at risk."

SOURCE: http://bit.ly/w2yW71 Journal of Clinical Psychiatry, online Oct. 4, 2011.