People who have trouble falling asleep may be at increased risk of developing cognitive problems or dementia than their counterparts who sleep well, a research review suggests.

Researchers examined data on 51 previously published studies that followed middle-aged and older people in North America, Europe and East Asia for at least several years to see if sleep issues were associated with cognitive health over time.

Individuals with insomnia were 27% more likely to develop cognitive problems, the study found. People who had what’s known as sleep inadequacy, or an insufficient amount of quality rest, were 25% more likely to develop dementia, the researchers also found.

So-called sleep inefficiency, or spending too much time wide awake in bed, was associated with a 24% greater chance of cognitive decline, the study team reports in Journal of Neurology, Neurosurgery, & Psychiatry.

“These findings suggested that sleep management might serve as a promising target for dementia prevention,” said lead study author Dr. Wei Xu of Qingdao University in China.

While the study wasn’t designed to determine whether or how sleep problems directly cause cognitive decline or dementia, there are several possible explanations, Xu said by email.

Sleep problems might lead to cognitive impairment by causing inflammation of tissue in the central nervous system including the brain and spinal cord, Xu said. Sleep difficulties might also lead cognitive problems by causing or exacerbating so-called cerebral hypoxia, or a reduction in oxygen supply in the brain, Xu added.

In addition, sleep problems could make the brain less efficient at removing waste and contribute to loss of brain cells or atrophy in key regions of the brain.

Most of the studies in the current analysis followed participants for anywhere from about three years to 10, and a few tracked people for decades. Participants were typically aged 50 or older at the start of these studies, and they were often in their 70s.

Sleep apnea, a nighttime breathing disorder, was associated with a 29% higher risk of cognitive problems, the analysis found. Spending a long time in bed was tied to a 15% greater chance of cognitive impairment.

People who slept around six or seven hours a night appeared to have the lowest risk of cognitive disorders, while the risk was elevated for people who slept less than four hours or more than 10 hours a night.

It’s possible that short sleep and other sleep problems contribute to degeneration in certain brain areas associated with dementia, the study authors note. Or, that people who already have such degeneration tend to sleep longer. Clinical trials are needed to test whether improving sleep quality and quantity affects later dementia risk, they write.

Matthew Pase, a researcher at the University of Melbourne in Australia who wasn’t involved in the study, agreed. “We still need more evidence to understand whether poor sleep leads to dementia and whether improving sleep can help mitigate risk.”

Nevertheless, Pase said by email, “sleep is vitally important for general health and should be made a priority.”

Most adults need at least seven hours of sleep a night, according to the U.S. Centers for Disease Control and Prevention.

Just because people struggle with sleep, however, doesn’t mean they’re destined to have cognitive problems, said Dr. Eric Larson of the Kaiser Permanente Washington Health Research Institute in Seattle.

“Insomnia and sleep complaints seem to cause a lot of anxiety that, in my judgment, is unwarranted if its source is brain health,” Larson, who wasn’t involved in the study, said by email. “But if sleep problems interfere with everyday activities and wellbeing during the day, that’s an issue that people should consider raising with their physician.”

Getting seven or eight hours of sleep a night also isn’t the only way to improve brain health, Larson said.

“The range of `healthy sleep amounts’ is highly variable between individuals and becomes more so as people age,” Larson said.

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