Getting too little -- or too much -- sleep could be bad for the aging brain, according to new studies presented at the Alzheimer's Association International Conference.
One of the studies in elderly women, conducted by researchers at Brigham and Women's Hospital, showed that those who slept fewer than five hours per day or more than nine hours per day had worse brain functioning than those who slept seven hours per day.
That study included 15,000 women ages 70 and older, whose brain functioning was first analyzed sometime between 1995 and 2000. The researchers continued to analyze their brain functioning every two years for a total of six years, and found that women who slept too much or too little had changes in brain functioning that were similar to aging two years.
The researchers found that when women slept for more or fewer than seven hours per day, it had an effect on an early sign of Alzheimer's disease, called the beta amyloid 42/40 ratio, compared with women who slept for seven hours a night.
They also found that if the amount of time a woman slept changed by two or more hours per day as she progressed from mid-life to later on in life, her brain functioning was worse, HealthDay reported.
"Our findings support the notion that extreme sleep durations and changes in sleep duration over time may contribute to cognitive decline and early Alzheimer's changes in older adults," study researcher Elizabeth Devore, Sc.D., said in a statement. "The public health implications of these findings could be substantial, as they might lead to the eventual identification of sleep- and circadian- based strategies for reducing risk of cognitive impairment and Alzheimer's."
Other research presented at the conference analyzed the risk of dementia based on sleep disorders. In that study, conducted by University of California, San Francisco researchers, people with sleep disorders like sleep apnea had a doubled risk -- if not more -- of dementia or mild cognitive impairment over a five-year period.
In another study, conducted by French researchers at INSERM, of 4,894 people, people who experienced excessive daytime sleepiness had a higher risk of declining brain functioning.
And in yet another study, researchers from the Washington University School of Medicine in St. Louis found a link between circadian rhythms and amyloid protein levels, HealthDay reported.
This is certainly not the first time disturbed sleep or sleep deprivation has been linked with brain and memory problems. Just last year, researchers from Stanford University published a study in the journal Proceedings of the National Academy of Sciences that showed that having constantly interrupted sleep is linked with an impaired ability to learn new things, the Los Angeles Times reported. That study was conducted in mice.
And earlier this year, a small study presented at the American Academy of Neurology showed that being woken up frequently at night and lying awake at night are linked with amyloid plaques, a sign of Alzheimer's disease, MedPage Today reported.
However, it's important to note that all of this research only notes a link, not a cause.
"Studies finding correlation are inadequate to confirm causality," Dr. Judy Willis, M.D., a neurologist, told MedPage Today, since it's not clear whether disturbed sleep brings on, or is the result of, disease.
Also on HuffPost:
More:Sleep Brain Too Much Sleep Alzheimers Too Little Sleep Alzheimers Poor Sleep Alzheimers Sleep Alzheimers
HuffPost Lifestyle is a daily newsletter that will make you happier and healthier — one email at a time. Learn more