Research shows that sleep helps people of all ages learn and remember.

Current theories suggest that slumber might help consolidate and stabilize memories, possibly by calming down overexcited synapses. But many questions still remain, including the roles of rapid-eye-movement (REM) and non-REM sleep.

Now, a study in the journal "PNAS" suggests naps might not benefit everyone equally, and lack of REM might be the reason.

Scientists taught preschoolers words for three objects, then tested how well they identified the objects after a 5-minute delay, after a 4-hour delay and after a nap. Confounding images were included to make the test harder.

The study group consisted of 24 typically developing children ages 2-4 and 25 children with Down syndrome ages 3-7. Each group was equally divided into males and females.

Down syndrome typically results from having three copies of chromosome 21 instead of two. Children with Down syndrome often experience sleep disorders, including apnea, insomnia and anxiety.

While typically developing children improved after a brief snooze, children with Down syndrome, who experienced less rapid-eye-movement (REM) sleep, showed learning losses.

"These results altogether provide further evidence for the benefits of napping for typically developing preschoolers, but also the possible mechanisms for poor memory skills in children with Down syndrome," said lead author Goffredina Spanò, who worked on the study while at University of Arizona.

Spanò, who now works as a postdoctoral fellow at University College London, hopes a larger study might bring the role of naps, and of REM sleep, more clearly into focus.

A project now underway at the UA will further test the effects of restricting naps in children with Down syndrome. UA researchers have previously investigated the effects of sleep apnea on that population.