Older women with memory problems are at increased risk for restless nights, say U.S. researchers who suggest cognitive decline may increase the chance of sleep problems.

Previous studies have found an association between sleep disturbances and poor cognitive function, but the latest research — to be published in Tuesday's edition of Neurology — raises the possibility that cognitive decline may increase the risk of sleep problems rather than vice versa, said lead author Dr. Kristine Yaffe.

"This study does not mean that an individual with cognitive problems will necessarily develop sleep trouble, but that these elders are at a higher risk of sleep disorders," Yaffe, of the University of California (San Francisco),said in a release on Monday.

"For women who declined on either cognitive test, they were nearly twice as likely to have difficulty staying asleep and one-and-half times as likely to have problems falling asleep and being awake for more than 90 minutes during their sleep cycle.

"Women who declined on one of the tests were also nearly twice as likely to nap more than two hours a day."

In the study, 2,474 women with an average age of 69 and no signs of memory problems underwent cognitive tests over the course of 15 years.

The researchers found the nearly 25 per cent of women who experienced cognitive decline were twice as likely as women without memory problems to suffer sleep disturbances.

The women's cognitive health was measured at regular intervals over the course of the study using two standard tests.

Sleep problems were measured at the end of the study. The women were fitted with an actigraph, a small device worn on the wrist that measures movement and is known from previous studies to be accurate in differentiating sleep from wakefulness.

The study found cognitive decline was not associated with total sleep time, but with the quality of sleep.

Yaffe suggests the most likely reason for the link between memory loss and sleep disturbances was that they share a common underlying cause, such as brain changes associated with Alzheimer's disease or other dementias.

Another reason could be that women with memory problems may also be anxious about their condition, which could affect their sleep, said Yaffe.

Yaffe and her colleagues plan to continue tracking sleep patterns and cognitive health over time among women of the same age group.

"Hopefully, we'll be able to tell if cognitive changes lead to sleep disturbances, or if the reverse is true, or if they have a common independent cause," she said.