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 than women who have a more robust circadian rhythm or are more physically active earlier in the day. That's the finding of a new study in the latest issue of the Annals of Neurology.

"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," says Greg Tranah, PhD., a scientist at the California Pacific Medical Center Research Institute - part of the Sutter Health network - and the lead author of the study. "What our findings suggest is 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, all over the age of 75, who were taking part in the Study of Osteoporotic Fractures. All the women 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 mild cognitive impairment (MCI). Those women 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," says 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," says 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."

In an accompanying commentary in the journal, Andrew Lim and Clifford Saper of the Department of Neurology at Harvard Medical School, say the study "represents a significant advance" in understanding the connection between circadian rhythm activity and dementia. "By showing that variations in rest-activity patterns precede the development of cognitive impairment and dementia, Tranah and colleagues have identified both a novel predictor of and a potential therapeutic target for incident cognitive deterioration and dementia."