USA Today interviewed Bill Thies, Chief Scientific and Medical Officer for the Alzheimer’s Association:

Q: What effect does sleep have on changes in the brain?

A: Researchers report that cognitive health declines over the long term in some people with sleep problems. A large study done at Brigham and Women’s Hospital in Boston, of 15,000 participants ages 70 and older, reported important findings. Sleep duration shorter than seven hours or longer than seven hours might have an effect on cognition in older individuals. It’s still not clear if sleep change is one of the risk factors or an early sign of Alzheimer’s.

Q: What else did the study note about sleep and cognition problems?

A: Participants who slept five hours or less had lower average cognition than those who slept seven hours a day. Those who slept nine hours a day or more had lower average cognition than those who slept seven hours per day. Both too little and too much sleep was cognitively equivalent to aging two years.