During deep sleep, older people have less coordination between two brain waves that are important to saving new memories, reports NPR.

“It’s like a drummer that’s perhaps just one beat off the rhythm,” says Matt Walker, one of the paper’s authors and a professor of neuroscience and psychology at the University of California, Berkeley. “The aging brain just doesn’t seem to be able to synchronize its brain waves effectively.”

The finding appears to answer a long-standing question about how aging can affect memory even in people who do not have Alzheimer’s or some other brain disease.