"It's thought that sleep apnoea could contribute to depression through its effects on sleep quality as sleep fragmentation occurs from the frequent apnoea events, but this hasn't been proven experimentally," said lead author Dr. Marcus Povitz of Western University in London, Ontario, Canada.
Read: Severe sleep apnoea aggravates blood pressure risk
"Also there is a hypothesis that the dips in oxygen due to the apnoea's might cause injury to the brain which could lead to depression," Povitz told Reuters Health by email.
Frequent waking and oxygen deprivation
Obstructive sleep apnoea (OSA) includes repeated upper airway obstruction during sleep - often due to the jaw sinking into the throat, closing off the airway - and, as a result, frequent waking and oxygen deprivation.
The problem has been linked to insomnia, poor memory and irritability as well as increased mortality and decreased quality of life, the authors write.
It may be caused by other health problems like congestive heart failure, but if not, it can be treated with gentle air pressure administered during sleep with a continuous positive airway pressure (CPAP) device, according to the Centres for Disease Control and Prevention.
For the review, the authors included randomised controlled trials of patients using CPAP devices or a mandibular advancement device (MAD), an alternative which holds the lower jaw and tongue forward during sleep.
They included 19 trials with CPAP machines, three with MADs, and two with both devices.
Read: Family docs can treat simple sleep apnoea
There was a wide range of study designs, but using CPAP devices did tend to improve depression symptoms compared to no apnoea treatment, and improved symptoms more if patients were more depressed at the start.
Treatment with MADs also improved depression symptoms compared to no treatment, according to the results published in PLoS Medicine.
Anecdotally, many sleep physicians have reported dramatic improvements in patient mood following sleep apnoea treatment, Povitz said, but the benefits they found in the new review were small, so more studies are needed to determine who will benefit the most.
"Approximately 6 percent of adult U.S. men and 3 percent of adult U.S. women have been told they had sleep apnoea by a doctor or other health professional," said Dr. Anne Wheaton of the CDC, who was not part of the new study.
Five percent of adults may have undiagnosed obstructive sleep apnoea syndrome with daytime sleepiness, and up to 20 percent have at least mild OSA, she told Reuters Health by email.
Sleep apnoea and depression often coexist
"It has been recognised for a long time that sleep apnoea and depression often coexist in the same patient, but nobody really knows the reason," said Dr. Richard Leung, assistant professor at the University of Toronto, who also was not part of the new study.
Sleep problems may be caused by depression or vice versa, he told Reuters Health by email. Sleep apnoea could also lead to symptoms that are similar to, and mistaken for, depression, he noted.
"I have treated sleep apnoea in patients with depression in my practice," Leung said.
"Following treatment, when they are feeling better, some choose to think of it as, 'My depression got better when my sleep apnoea got treated'," he said. "Others choose to frame it as, 'I never had depression at all. It was always sleep apnoea.' I don't know which of them is right."
The new review indicates that sleep apnoea may at least partially contribute to the development or worsening of depression, Wheaton said.
Sleepiness is a symptom of depression
"Feeling excessively sleepy or tired is in and of itself a symptom of depression," Wheaton said. "If treating sleep apnoea improves sleepiness, that is already a benefit."
Sleep apnoea also increases the risk of high blood pressure, diabetes and cardiovascular disease. Post-menopausal women and people who are obese are at higher risk for sleep apnoea, she said.
Dr. Karim Ghobrial-Sedky, associate professor of psychiatry at Cooper Medical School of Rowan University in Camden, New Jersey, recently had a patient with hypertension resistant to treatment, and after screening for and treating severe obstructive sleep apnoea, the person's blood pressure was better controlled.
Read: Can sleep apnoea affect your sex life?
"Educating physicians and patients to recognize this disorder is important," Ghobrial-Sedky told Reuters Health by email.
"Exploring different methods to decide on the most appropriate treatment for each individual is important," he said. "Emphasizing the risks of not treating the disorder for patients is also essential."
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Image: Man with lack of sleep from Shutterstock.