They learned that by monitoring brain activity in rats that had spent the day learning how to scamper around a circular arena in a particular pattern.
While the rats slept, their medial prefrontal cortex had spurts of activity that matched brain activity while the rats had been running around the arena earlier in the day.
The researchers -- who included David Euston, PhD -- took that as a sign that the rats' brains were replaying the experience of learning the route around the arena.
But during sleep, those brain patterns happened six to seven times faster than the time it had taken the rats to scurry around the arena while awake.
In short, the rats' brain activity kicked into overdrive during sleep to consolidate memories.
Scientists already knew that another part of the brain -- the hippocampus -- helps make memories during sleep.
The University of Arizona study shows that the medial prefrontal cortex is also involved in making memories during sleep. The findings appear in tomorrow's edition of Science.