People who have more disrupted sleep in their 30s and 40s may be more likely to have memory and thinking problems a decade later, according to new research published in the online issue of Neurology, the medical journal of the American Academy of Neurology.

The study, however, does not prove that sleep quality causes cognitive decline. It only shows an association.

“Given that signs of Alzheimer’s disease start to accumulate in the brain several decades before symptoms begin, understanding the connection between sleep and cognition earlier in life is critical for understanding the role of sleep problems as a risk factor for the disease,” said study author Yue Leng, from the University of California, San Francisco. “Our findings indicate that the quality rather than the quantity of sleep matters most for cognitive health in middle age.”

The study involved 526 people with an average age of 40. They were followed for 11 years.

Researchers looked at participants’ sleep duration and quality. Participants wore a wrist activity monitor for three consecutive days on two occasions approximately one year apart to calculate their averages. Participants slept for an average of six hours.

Participants also reported bedtimes and wake times in a sleep diary and completed a sleep quality survey with scores ranging from zero to 21, with higher scores indicating poorer sleep quality. A total of 239 people, or 46%, reported poor sleep with a score greater than five.

Participants also completed a series of memory and thinking tests.

Researchers also looked at sleep fragmentation, which measures repetitive short interruptions of sleep. They looked at both the percentage of time spent moving and the percentage of time spent not moving for one minute or less during sleep. After adding these two percentages together, researchers found that participants had an average sleep fragmentation of 19%.