A study reported at the Alzheimer's Association International Conference® 2012 showed that sleep disorders and circadian disruptions are associated with increased risk of cognitive impairment.

As people age, they are more likely to develop problems with sleeping, such as insomnia, sleep apnea and disruptions in circadian rhythm. Whether these problems affect one's cognitive function or risk of developing mild cognitive impairment (MCI) or dementia is not definitively known.

"Studies that have explored the relationship between sleep and dementia are often cross sectional and depend on the participant's self-report rather than objective measures of sleep quality," said Kristine Yaffe, M.D., of the University of California, San Francisco. 

Instead, Yaffe and colleagues conducted a series of studies evaluating more than 1,300 older women (≥75) enrolled in a large multi-center study to investigate the relationship between objectively measured sleep quality (using actigraphy and polysomnography) and adverse cognitive outcomes.

Over five years, the researchers assessed cognitive function and clinical cognitive status (normal, mild cognitive impairment [MCI] or dementia) and obtained objective measures of sleep parameters, including sleep apnea, nighttime wakefulness, total sleep time, and shifts in circadian rhythm.
The scientists found that:

• Participants with sleep-disordered breathing or sleep apnea had more than twice the odds of developing MCI or dementia over the five years compared with those who did not have sleep-disordered breathing.
• Women who developed a disruption of their circadian rhythm (delay in the acrophase) over the five years were at increased risk of developing MCI or dementia compared with individuals who did not.
• Participants with greater nighttime wakefulness were more likely to score worse on tests of global cognition and verbal fluency than those without it.

"We believe that these results indicate that the relationship between sleep disordered breathing and dementia may be connected to the decrease in oxygen associated with sleep apnea and not to disrupted patterns of sleep," Yaffe said. 

"Overall, our findings support a relationship between sleep disturbances and cognitive decline in late age. They suggest that health practitioners should consider assessing older people with sleep disorders for changes in cognition," Yaffe said. "In addition, with additional long-term research, treatment of sleep disorders may be a promising method of delaying the development of MCI and dementia."