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A team of researchers from Australia, the United States, and Canada has found that the third stage of sleep—known as slow-wave sleep—plays a crucial role in reducing the risk of dementia. According to their study, individuals over the age of 60 are 27 percent more likely to develop dementia if they lose just 1 percent of their slow-wave sleep each year.
Previous studies show deep sleep helps strengthen muscles and bones, supports the immune system, and prepares the brain to absorb new information.
Slow-wave sleep is the third stage of the human sleep cycle; it typically begins about 40 minutes after falling asleep and lasts roughly 20 to 40 minutes. It’s the deepest, calmest phase of sleep, when brain waves and heart rate slow and blood pressure falls. Sleep scientists distinguish Stage 1 (the lightest stage, when someone can wake up quickly), Stage 2 (light NREM sleep, which makes up nearly half the cycle), Stage 3 (slow-wave deep sleep), and REM sleep (the stage when we dream).
What Scientists Discovered
The researchers conclude that the risk of developing dementia increases with age when people lose slow-wave sleep.
“Slow-wave or deep sleep protects the brain from aging, enhances the removal of metabolic waste from the brain, and helps eliminate proteins that aggregate in Alzheimer’s disease,” said neurobiologist Matthew Pace, the lead author of the study at Monash University in Australia.
Pace’s team studied sleep data from 346 volunteers between 1995 and 1998 and again from 2001 to 2003.
During the 2001–2003 recordings, the researchers did not document any cases of dementia among participants. But by 2020, when the volunteers were over 60, the team found intriguing results, as reported by Science Alert. They first examined how slow-wave sleep changed in participants as they aged, then linked those changes to the development of dementia in later life.
The team found that slow-wave sleep began to decline significantly after age 60. The decline was steepest between 75 and 80, then leveled off.
After 2003, 52 cases of dementia were recorded among the participants.
They found that each 1 percent annual reduction in slow-wave sleep was associated with a 27 percent higher risk of developing dementia. For Alzheimer’s disease, the most common form of dementia, the risk rose by 32 percent.
The study also linked poor slow-wave sleep quality with higher rates of cardiovascular disease, the use of sleep-disrupting medications, and the presence of the APOE ε4 gene, which is associated with Alzheimer’s disease.
The study’s findings were published in JAMA Neurology.
