Disrupted sleep in one's 50s, 60s raises Alzheimer's risk
A study involving 95 healthy older adults found that adults reporting a decline in sleep quality in their 40s and 50s had more amyloid-beta in their brains later in life, while those reporting poorer sleep in their 50s and 60s had more tau tangles. Those with high levels of tau protein were more likely to lack the synchronized brain waves during deep NREM sleep that are associated with a good night's sleep, and the more tau protein, the less synchronized these brain waves were.
Previous research has found that a dip in the amplitude of slow wave activity during deep NREM sleep was associated with higher amounts of beta-amyloid in the brain and memory impairment.
Studies of healthy animals and humans have reported higher levels of amyloid beta after a single night of sleep deprivation, and that disruption of slow-wave sleep causes amyloid beta levels to rise as much as 30%. Moreover, a single night’s sleep deprivation has been found to increase tau levels by as much as 50% in cerebrospinal fluid.
These findings suggest that quality sleep helps the body clear excess amyloid and tau proteins.
A preliminary study involving 20 healthy subjects aged 22 to 72 found beta-amyloid increases of about 5% after losing a night of sleep.
Many researchers believe the link between sleep disorders and Alzheimer's risk is "bidirectional," since elevated beta-amyloid may also lead to sleep disturbances.
A very small study involving eight people aged 30-60, who experienced (over time) two or three different sleep situations, found that amyloid beta levels were 25-30% higher when individuals had a a sleepless night — putting those amyloid beta levels on par with the levels seen in people genetically predisposed to develop Alzheimer’s at a young age.
A sleep study involving 17 healthy adults aged 35 to 65, found that those whose slow-wave sleep was disrupted (by a beeping sound that moved them into a shallower sleep) found a 10% increase in amyloid beta levels after a single night of interrupted sleep, but no corresponding increase in tau levels. However, participants whose activity monitors showed they had slept poorly at home for the week before showed a spike in levels of tau.
Is too much sleep an early sign of dementia?
Data from 2,457 older adults (65+) in the Framingham study found that those who consistently slept more than nine hours every night had twice the risk of developing dementia and Alzheimer’s disease within the next 10 years, compared to those who slept less than nine hours a night.
Over the 10-year study period, 234 were diagnosed with dementia.
It’s suggested that one reason might be that those with depression tend to sleep longer. In any case, it’s thought that the longer sleep sessions reflect something else going on, rather than being a cause.
Education level also affected the degree of risk. Those without a high school degree who slept more than nine hours nightly had a 600% greater risk of later receiving a dementia diagnosis than people with a high school degree.
Optimal sleep linked to lower dementia risk
A ten-year Japanese study involving 1,517 older adults (60+) found that dementia rates were higher in those with daily sleep duration of less than 5 hours or more than 10 hours, compared with those with daily sleep duration of 5-6.9 hours. However, those with short sleep duration who had high physical activity did not have a greater risk of dementia.
294 participants (19%) developed dementia in the 10 year period.
Lack of REM sleep linked to higher dementia risk
A study involving 321 older adults (60+; average age 67), who participated in a sleep study between 1995 and 1998, found poorer REM sleep was associated with an increased risk of developing dementia over 12 years.
During that period, 32 people were diagnosed with some form of dementia (24 with Alzheimer’s)
Those who developed dementia spent an average of 17% of their sleep time in REM sleep, compared with 20% for those who didn’t develop dementia. For every percent that REM sleep was reduced, there was a 9% increase in dementia risk, and an 8% increase in Alzheimer’s risk specifically.
No such associations were found for other stages of sleep, although that shouldn’t be taken to mean that other sleep stages don’t affect key features of Alzheimer’s.
Sleep apnea linked to higher tau levels
A study involving 288 cognitively healthy older adults (65+) found that those who had sleep apneas had on average 4.5% higher levels of tau in the entorhinal cortex than those who did not have apneas, after controlling for several other factors that could affect levels of tau in the brain, such as age, sex, education, cardiovascular risk factors and other sleep complaints.
15% (43 participants) were reported by their bed partners as having sleep apneas.
This preliminary study was presented at the American Academy of Neurology's 71st Annual Meeting in Philadelphia, May 4-10, 2019.
Data from 1,752 older adults found that sleep-disordered breathing was associated with poorer attention and processing speed. In particular, increased overnight hypoxemia (oxygen saturation below 90%) was linked with poorer attention and memory, and more daytime sleepiness associated with poorer attention and memory and slower cognitive processing speed.
These associations were strongest in APOE-ε4 carriers.
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