When we sleep, fresh memories become converted into long-term ones, according to researchers at the University of Arizona. But the older we get, the harder it is for our brains to make that unconscious conversion.

The team of scientists studied the brain activity of 22 rats—11 young and 11 old—to determine their ability to store new information. After a long day of maze wandering, the younger rats replayed the events of the day in the hippocampal region of the brain. The majority of the older rats did not replay the information, indicating that the new memories were not being properly stored during sleep.

A reduction in the brain’s experience-replay function during sleep could be behind age-related memory decline, researchers say. The results of their study can be found in the July 30 issue of The Journal of Neuroscience.