Poor sleep has been linked to the development of dementia and Alzheimer's disease, and now a new study suggests a possible reason why.
A small group of young, healthy men deprived of just one night of sleep had higher blood levels of tau protein than when they had a full and uninterrupted night of rest, researchers reported in a study published online in Neurology.
Protein linked to Alzheimer's
"This is interesting as accumulation of the protein tau is seen in the brains of individuals afflicted by Alzheimer's disease, or most common forms of dementia," said senior study author Dr Jonathan Cedernaes, a senior researcher at Uppsala University in Sweden.
The researchers did not find any similar increase in amyloid beta, another brain protein long linked to Alzheimer's, the Swedish researchers said.
The new findings come as Alzheimer's research has started to shift its focus towards tau as a more important cause of brain damage associated with the disease.
Another group of researchers recently reported in the journal Science Translational Medicine that they can predict with reasonable accuracy which brain regions will wither and atrophy in Alzheimer's by identifying the places where tau protein "tangles" have accumulated.
Tau is a protein found in normally active neurons, and it is typically cleared from the brain rapidly, Cedernaes said.
But in Alzheimer's patients, tau sticks together to form tangles that linger in the brain.
Increase in tau levels
For this latest study, researchers recruited 15 men with an average age of 22, all of whom said they regularly get seven to nine hours of quality sleep each night.
The researchers observed each man through two sets of two-day sleep cycles. In the first cycle, they got two good nights of sleep, but in the second cycle they were deprived of sleep for one night.
The men had an average 17% increase in tau levels in their blood after a night of sleep deprivation, compared to an average 2% increase in tau levels after a good night of sleep, the findings showed.
No other biomarkers associated with Alzheimer's showed a similarly significant change, including amyloid beta, according to the report.
At this point, scientists don't know why tau might build up in the bloodstream of people who've had bad sleep, Cedernaes said.
'Trash removal' interrupted
"When neurons are more active, they secrete more tau," Cedernaes said. "It may be that when we remain awake for extended periods of time – much longer than the 15 to 18 daily hours that we are supposed to – then this increases levels of tau in the brain to a point where it exceeds the brain's ability to clear it effectively for a given 24-hour period."
Sleeplessness might also affect the way in which the body clears tau protein from the brain, said Keith Fargo, director of scientific programmes and outreach at the Alzheimer's Association.
"Everyone produces amyloid and tau in their brain every day, and the brain is supposed to take out the trash," Fargo said. "The thinking is if the sleep is disrupted in some way, the processes involved in taking out the trash are disrupted."
Another possibility is that tau is released from brain cells when they are damaged, Fargo added. For example, head trauma can increase blood levels of tau.
"If a brain cell dies, the tau can spill out of the brain cell," Fargo explained. "Maybe what you're seeing is a loss of brain cell integrity if you're not sleeping."
Although the findings are interesting, Fargo noted that this was a very small study involving only young people.
More research needed
Larger studies involving middle-aged folks who have poor sleep for more than just one night – perhaps a week or a month – would provide even better data that could be more closely tied to dementia and Alzheimer's, Fargo said.
Cedernaes agreed that more research is needed to come to a better understanding of this association.
"At present, we do not know exactly what these changes represent, nor do we have any data indicating that a single or even multiple nights of sleep loss lead to some permanent harmful event in the brain," Cedernaes said. "Many individuals are forced to undergo repeated bouts of sleep loss and maintain perfect cognition throughout life."
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