Studies reported at the Alzheimer's Association International Conference® 2012 (AAIC® 2012) suggest a relationship between sleep quality and quantity and risk of cognitive decline, and that interventions to normalize sleep duration and correct sleep disorders may not only improve quality of life, but have potential to reduce or prevent cognitive decline.

"We know that sleep patterns change as people age and that poor sleep affects overall health. What we don't know for certain is whether poor sleep has long-term consequences on cognitive function," said William Thies, Ph.D., Alzheimer's Association® chief medical and scientific officer.

"The studies presented at AAIC suggest that cognitive health declines over the long term in some people with sleep problems. The good news is that tools already exist to monitor sleep duration and quality and to intervene to help return sleep patterns to normal. If we do this, there is the possibility that we may also help people preserve their cognitive health, but that needs to be tested," Thies added.

One study showed that too little and too much sleep are associated with lower cognition.  Growing evidence suggests that sleep duration that is shorter or longer than the recommended seven hours per day may increase risk of cardiovascular disease and type 2 diabetes. However, little research has been conducted examining whether sleep duration influences cognition among older individuals.

Elizabeth Devore, Sc.D., of Brigham and Women's Hospital, Boston, and colleagues examined data for more than 15,000 participants in the Nurses' Health Study. All were age 70 or older at their first cognitive examination between 1995 and 2000. Follow-up cognitive assessments were conducted every other year for six years. Participants' daily sleep duration was categorized as ≤ 5, 6, 7, 8 or ≥9 hours (7 hours per day was considered normal). Average sleep duration was self-reported in 1986 (when women were ages 40 to 65) and 2000 (when women were ages 54 to 79). 

The scientists found that:

• Participants who slept 5 hours per day or less had lower average cognition than those who slept 7 hours per day.
• Those who slept 9 hours per day or more also had lower average cognition than those who slept 7 hours per day.
• Too little or too much sleep was cognitively equivalent to aging by 2 years.

When the researchers evaluated the effects of change in sleep duration from mid- to later- life, they observed that women whose sleep changed by 2 hours per day or more had worse cognitive function than those with no change in sleep duration, independent of their initial sleep duration. 

To explore sleep duration in relation to an early biomarker of Alzheimer's, the scientists examined the association with plasma levels of a ratio of proteins indicative of Alzheimer's disease brain changes (beta amyloid 42/40 ratio), which was measured in a small subset of women who provided blood samples in 1999-2000. They found that sleep durations of more than 7 or less than 7 hours per day were associated with declining ratios of Aβ42/40 compared to sleep durations of 7 hours per day.

"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," Devore said. "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."