Taking a mid-afternoon nap may prepare the brain to learn new things, early research suggests.

Researchers in the U.S. studied 39 young adults who were divided into two groups. At noon, study participants took a memory test that required them to remember faces linked to names.

Of those in the study, 20 took a nap for 100 minutes. All of the volunteers were then retested at 6 p.m.

Those who stayed awake did about 10 per cent worse on the tests compared with those who napped, Matthew Walker of University of California at Berkeley said. He presented the preliminary findings Sunday at the American Association of the Advancement of Science meeting in San Diego.

The more hours we spend awake, the more sluggish the brain becomes, the study suggests.

Normally, the ability to learn declines between noon and 6 p.m., but a nap seemed to fight off the decline.

"After about 1:30, I notice my last class of the day I just want the day to be over," Marquis Majore said during reading week at the University of Regina. "It's hard to concentrate and stay focused in class."

Previous data from the same team showed pulling an all-nighter also reduces the brain's ability to cram in new facts by nearly 20 per cent.

Walker's team showed that fact-based memories are temporarily stored in a region of the brain called the hippocampus before being sent to the brain's prefrontal cortex, which may have more storage space.

Clearing brain's inbox

"It's as though the email inbox in your hippocampus is full and, until you sleep and clear out those fact emails, you're not going to receive any more mail. It's just going to bounce until you sleep and move it into another folder," Walker said in a statement.

The key is to sleep long enough for the brain to progress through its sleep cycles.

When tests were used to track electrical activity in the brain, the team found the refreshing effects of sleep seemed to occur between deep sleep and the dream state, known as rapid eye movement or REM.

The challenge is to find 90 minutes during the day to nap, said Kawaku Adu, a law student at the University of Regina.

"I hardly sleep during the daytime," Adu said. "I'm usually in school, I'm in class or in the library."

The new results could help explain why humans spend at least half of their sleeping hours in non-REM sleep.

A larger study is planned to investigate whether the reduction in sleep that people experience as they get older is related to the known decline in ability to learn as people age.