New research finds that extreme sleep durations and changes in sleep duration over time may contribute to cognitive decline and early Alzheimer’s changes.

Elizabeth Devore, ScD, 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 6 years. Participants’ daily sleep duration was categorized as less than or equal to 5, 6, 7, 8, or greater than or equal to 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 amyloid-beta 42/40 compared to sleep durations of 7 hours per day.

"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,” Devore said.

The research was reported at the

Alzheimer’s Association International Conference