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, 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 six 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.

“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.”

Sleep disorders, circadian disruptions 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. (Circadian rhythms are physical, mental and behavioral changes that follow a 24-hour cycle.) 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, MD, of the University of California, San Francisco.

Instead, Yaffe and colleagues conducted a series of studies evaluating more than 1,300 older women (greater than or equal to 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.

* Actigraphy is a method of monitoring rest/activity cycles. A small sensor unit is worn, usually on the wrist, to measure gross motor activity. The unit continually records the movements it undergoes.

** Polysomnography, also known as a sleep study, is a comprehensive recording of the physical changes that occur during sleep. The PSG monitors many body functions including brain, eye movements, muscle activity or skeletal muscle activation, breathing functions, and heart rhythm.

Over five years, the researchers assessed cognitive function and clinical cognitive status (normal, 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.”

Sleep complaints and risk of cognitive decline in the elderly

Sleep quality and quantity often decrease as people age, but whether this is associated with risk of cognitive decline is a subject of ongoing investigation.

To better understand the potential relationship between sleep and cognitive decline, Dr. Claudine Berr of INSERM, Montpellier, France, and colleagues examined data from the French Three-City Study, an ongoing, long-term, multisite study of the relationship between vascular disease and dementia in community-dwelling individuals age 65 or older. Cognitive data were obtained at baseline and at 2, 4, and 8 years’ follow-up; an ancillary project gathered data on sleep complaints at baseline.

Researchers analyzed data from 4,894 nondemented study participants who had completed sleep questionnaires, had Mini Mental State Examination (MMSE) scores of greater than or equal to 24 at baseline, and for whom data from at least one follow-up evaluation was available. Cognitive decline was defined as a decrease of four or more points on the MMSE at the three follow-up points. Researchers analyzed data for each of the five subcomponents of insomnia: (1) poor sleep quality, (2) difficulty initiating sleep, (3) difficulty maintaining sleep, and (4) early morning awakening; and excessive daytime sleepiness.

They found that excessive daytime sleepiness, which was reported by 17.9 percent of participants, independently increased risk of cognitive decline. In contrast, difficulty maintaining sleep, reported by 63.5 percent of participants, was negatively associated with risk of cognitive decline.

“These results suggest that excessive daytime sleepiness may be an early predictor of cognitive decline and that sleep complaints should be adequately evaluated in older persons,” said the study’s researchers.

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