WASHINGTON - Getting six to nine hours of sleep per night is associated with higher ratings for quality of life and lower ratings for depression, suggests a research abstract that was presented Tuesday at SLEEP 2011, the 25th Anniversary Meeting of the Associated Professional Sleep Societies LLC in Minneapolis, Minnesota.

Results show that people with a "normal" sleep duration of six to nine hours per night had higher self-reported scores for quality of life and lower scores for depression severity compared to short and long sleepers.

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These differences were statistically significant in all comparisons. Among patients who reported having perfect health, there was a higher percentage of normal sleepers, who also had significantly lower scores for depression severity compared with short and long sleepers with perfect health.

"These results are important because they provide more information about the importance of getting enough sleep, which is usually six to nine hours per night," said principal investigator Charles Bae, neurologist at the Cleveland Clinic Sleep Disorders Center in Ohio.

"People may already expect that their quality of life could be decreased when they do not get enough sleep, but they may not realize that sleeping too much can also have a negative impact," he added.

Bae and colleagues analyzed data from 10,654 patient records, which were collected from January 2008 to May 2010. Study subjects had a mean age of about 52 years. Quality of life was assessed using the EQ-5D questionnaire, a standardized measure of health outcome. The nine-item Patient Health Questionnaire was used as a screening tool for depression.

Generalized estimating equations were used to account for multiple visits per patient, and a multi-variable logistic regression model adjusted for demographic differences such as age, gender, race and marital status. Short sleep was defined as less than six hours per night, and long sleep was classified as more than nine hours per night.

"It was surprising to see that sleeping less than six hours and more than nine hours is associated with a similar decrease in quality of life and increase in depressive symptoms," said Bae. "I thought that there would be changes in quality of life and degree of depressive symptoms for short and long sleepers, but did not expect that those changes would be similar in both groups."

The American Academy of Sleep Medicine reports that individual sleep needs vary. However, most adults need about seven to eight hours of nightly sleep to feel alert and well rested during the day.