After making adjustments for differences in the participants' physical activity, weight and blood pressure -- which have been shown to affect sleep quality -- the researchers compared changes in participants' brain scans and reported their findings online Sept. 3 in Neurology.
In those with poor sleep quality, the researchers saw shrinkage in one part of their frontal cortex and some atrophy, or deterioration, throughout three other parts of the brain, including parts involved with reasoning, planning, memory and problem-solving.
The study didn't test participants' thinking skills, so it couldn't prove that poor sleep or brain shrinkage was linked to poor memory or difficulty thinking. However, past research has found links between declining memory and decreases in brain volume.
"We often correlate brain shrinkage with losing brain tissue, and assume that that isn't advantageous as you get older," said Anton Porsteinsson, director of Alzheimer's disease care, research and education at the University of Rochester School of Medicine and Dentistry in New York.
"Sleep disturbance is such a common symptom among the general population, and it often becomes worse as you age," he said. "There is growing data to suggest that sleep disturbance may be a risk factor for poor outcomes in terms of brain cells and other medical issues as well."
The correlation was only with poor quality of sleep, not shorter sleep. The reduced brain size in poor sleepers was seen across all ages, but the correlation was stronger among adults over 60, the study found.
"What this study signals to me is that [good bedtime habits] and good sleep matters," Porsteinsson said.
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