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Older women with weaker circadian rhythms who are less physically active or are more active later in the day are more likely to develop dementia or mild cognitive impairment (MCI) than women who have a more robust circadian rhythm or are more physically active earlier in the day, a new study suggests (Annals of Neurology, December 2011).
"We've known for some time that circadian rhythms - what people often refer to as the "body clock" - can have an impact on our brain and our ability to function normally," said lead author Greg Tranah, PhD, a scientist at the California Pacific Medical Center Research Institute, part of the Sutter Health network, in San Francisco. "Our findings suggest that future interventions such as increased physical activity or using light exposure interventions to influence circadian rhythms could help influence cognitive outcomes in older women."
The researchers collected data on activity and circadian rhythm from 1,282 healthy women over age 75 who were taking part in the Study of Osteoporotic Fractures. They underwent a series of neuropsychological tests to ensure they had no evidence of cognitive or brain problems. At the end of five years, 15 percent of the women had developed dementia, and 24 percent had some form of MCI. Those who had weaker circadian rhythm activity, lower levels of activity, or whose peak level of activity was later in the day were at highest risk of developing dementia or MCI.
"This was not a small difference but a rather sizable, statistically significant one," said Dr. Tranah. "Those who had the later wake times, whose activity was later in the day, were 80 percent more likely to develop MCI or dementia compared to women who had earlier wake times and earlier activity."
Circadian rhythms play an important role in the control of sleep-wake cycles, and there is considerable evidence to show they also play a role in regulating certain brain functions, such as alertness, learning and memory. As people get older, the activity level of those rhythms - how strong they are - often change, bringing with it changes in sleep patterns and levels of physical activity.
"To our knowledge this is the first study to show such a strong connection between circadian activity rhythm and the subsequent development of dementia or MCI," said Dr. Tranah. "The reasons why this is so are not yet clear. The changes in circadian rhythm may directly influence the onset of dementia or MCI, or the decrease in activity may be a consequence - a warning sign, if you like - that changes are already taking place in the brain. Identifying what the reason is could help us develop therapies to delay or slow down the development of brain problems in the elderly."
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