Taking longer to drift into the dreaming phase – known as rapid eye movement (REM) sleep – could be an early sign of Alzheimer's disease, potentially opening up new ways to diagnose and treat the devastating condition before it fully develops.
The findings by a team from China, the US, and Spain build on previous work looking at the relationships between sleep quality, memory formation, cognitive thinking, and dementia risk.
"Sleep disturbances are common in patients with dementia and have been associated increasingly with Alzheimer's disease, and Alzheimer's disease and related dementias biomarkers, even in preclinical stages of the disease," write the researchers in their published paper.
To analyze this further, 128 adults with an average age of nearly 71 stayed overnight in a sleep clinic where their slumber could be monitored. Of these, 64 had been diagnosed with Alzheimer's, and 41 had mild cognitive impairment.
The researchers split the sleepers up into groups, depending on how long it took them to reach REM sleep. For the group that hit REM quickest, this was less than 98.2 minutes; for the group that took the longest, it was over 192.7 minutes.
After adjusting for factors such as age, cognitive function, and genetic risk, the researchers found those in whom REM was most delayed were more likely to have Alzheimer's than those who were earliest to reach the sleep stage.
Further comparisons showed the late REM group tended to have more of the amyloid beta and tau protein buildup associated with Alzheimer's in their brain. They also tended to have less of a protein that supports neuron survival and helps our brains learn, called brain-derived neurotrophic factor (BDNF).
It doesn't mean direct cause and effect, but it's a useful indicator. The researchers think that reaching REM earlier leads to a chemical balance in the brain that's healthier and better protected against the ravages of dementia.
"The delay in REM sleep disrupts the brain's ability to consolidate memories by interfering with the process that contributes to learning and memory," says epidemiologist Yue Leng, from the University of California, San Francisco.
"If it is insufficient or delayed, it may increase the stress hormone cortisol. This can impair the brain's hippocampus, a critical structure for memory consolidation."
As ever, it's a complicated picture with Alzheimer's. It's tricky to pick out the causes of the disease from its consequences – as is the case with the toxic aggregations of the amyloid beta and tau proteins, which might be triggering Alzheimer's or appearing as a result of it.
However, with each study like this we get a better understanding. The researchers suggest that promoting a healthy sleep cycle – by treating conditions like sleep apnea and avoiding heavy boozing, for example – could help avoid REM sleep delays, support memory formation, and ultimately reduce dementia risk.
"Future research should study the effects of certain medications that influence sleep patterns, as these may modify disease progression," says Leng.
The research has been published in Alzheimer's and Dementia: The Journal of the Alzheimer's Association.