Washington: It's already known that a good night's sleep helps in memorising but now, a new study has found that having a nap before you learn is important too.

Researchers at the University of California, Berkeley, found that volunteers who took a 100-minute nap before an memorisation task scored on average 20 per cent higher that those who participated in the task without snoozing first.

"It really seems to be the first evidence that we're aware of that indicates a proactive benefit of sleep," study co-author Matthew Walker, a professor of psychology and neuroscience, said.

"It's not simply enough to sleep after learning. It turns out you also need to sleep before learning," Walker told LiveScience.

Earlier research has found that dreams boost learning, with one study suggesting a 90-minute nap may help lock in long-term memories.

But Walker's research, published in the journal Current Biology, found that another phase of sleep, called nonrapid eye movement (NREM), is most closely linked to the learning boost provided by a nap.

For their study, the researchers recruited 44 volunteers who were first given a task to memorise 100 names and faces and then tested for how well they recalled them.

Next, the researchers tucked half of the volunteers in for a 100-minute nap in the afternoon. The scientists measured the napping volunteers' brain waves as they slept.

The other group of participants stayed awake and did daily activities as they normally would. At 6 pm, both groups memorised another set of 100 faces and names and were tested on their memory.

The first major finding, Walker said, was that learning ability degrades as the day wears on. Volunteers who didn't nap did about 12 per cent worse on the evening test than they did on the morning test.

But shut-eye not only reversed those effects, it gave a memory boost: Napping test-takers did about 10 per cent better on the evening test than they did on the morning test.

In all, the difference in scores between non-nappers and nappers was about 20 per cent, Walker said.

According to the scientists, a short, synchronised burst of electrical activity called a sleep spindle plays a crucial role in boosting memory. The spindle lasts about one second and can occur 1,000 times per night during NREM sleep.

People who had more of these spindles showed the most refreshment in learning capacity after their nap, Walker said.

The researchers believe that the sleep spindles are working to transfer information from the hippocampus --a small area deep in your brain where memories are made -- to the prefrontal cortex, which serves as long-term storage. That frees up the hippocampus to make new memories, Walker said.

"It's almost like clearing out your informational inbox of your e-mail so you can start to receive new e-mails the next day," he said.

NREM sleep and sleep spindle frequency change throughout a person's life span. Older people, Walker said, have a decline in sleep spindles, suggesting that sleep disruption could be one reason for the memory loss prevalent in old age.

The volunteers in the current study were young, but the researchers hope to investigate the effect of sleep spindles on learning in older adults, Walker added.