Sleep problems are often a symptom of depression, but a new study raises the possibility that they could cause depression as well.
In a sample of nearly 2,000 Australian men between the ages of 35 and 83, those with excessive daytime sleepiness were 10% more likely to be depressed than those without, according to researchers from the University of Adelaide and the Adelaide Institute for Sleep Health. The relationship held up even after taking other risk factors into account.
None of the men had been diagnosed with severe obstructive sleep apnea when they entered the study, but 857 of them were assessed for the condition after joining. Those who were found to have it were 2.1 times more likely to be depressed than those who didn't have the sleep disorder.
Some of the men had both severe sleep apnea and excessive daytime sleepiness. They were 4.2 times more likely to be depressed compared with men who had no sleep issues, the researchers found. Those with both conditions were also 3.5 times more likely to be depressed than men with only one of them.
All of the men in the study were evaluated for depression twice, with the second test occurring about five years after the first. That allowed the researchers to see whether sleep problems could be linked to a recent diagnosis of depression.
And indeed, the men who had severe sleep apnea that was discovered during the study were 2.9 times more likely to become depressed during those five years.
The study design did not allow researchers to determine whether sleep problems boosted the men's risk of depression. It's possible that the reverse is true, or that a third factor makes people more likely to be both depressed and to have trouble sleeping.
Though the nature of the link between sleep and depression is still hazy, the results do provide actionable information for doctors, the study authors concluded. After patients are diagnosed with depression, they should be screened for obstructive sleep apnea, even if they don't seem to be sleepy, they wrote.
Their results were presented this week at the American Thoracic Society's 2015 International Conference in Denver.