Sleeping and napping are good for memory, say two new US studies — and short naps are best of all, says an Australian expert.

Two studies from Harvard University advocate sleep for improved mental performance.

The first, published in this month's Nature Neuroscience, found that mental performance improves with a nap in the middle of the day.

But Dr Saul Gilbert, from the Centre for Sleep Research at the University of South Australia, was surprised at the length of naps recommended — 30 minutes to one hour.

"Power naps are only beneficial if they are 10 to 15 minutes long," he said.

"If you sleep for longer you wake up groggy."

The US researchers waited one hour after study participants woke up from a nap before asking them to complete the test tasks, and this is where the results are a little misleading, cautioned Dr Gilbert.

"That time allows the subjects to get over their sleep inertia."

Previous Australian research has shown that shorter naps are beneficial because they are too short to go into slow wave (restful and deep) sleep.

"If you want a quick-pick-me up to make your general performance better, it's better after a 10 to 15-minute nap than if you spend 10 to 15 minutes doing nothing," Dr Gilbert explained. "And it's certainly better than if you sleep for half an hour or more."

The US researchers postulated that sleep allows information to be consolidated to memory. Dr Gilbert agrees with this theory.

"I think of it as the brain defragging itself, like a computer," he said.

Having a longer nap means the subjects were falling into a stage of deep sleep called 'slow-wave sleep', which the US researchers believe to be responsible for memory consolidation.

Sleep has five stages — 1, 2, 3, 4, and REM (rapid eye moment) sleep. Brain activity during each stage is measured by attaching a series of electrodes to the subject's face and head.

Stage 1 is the 'nodding off' stage people often experience on the bus or in meetings. Stage 2 is indicated by periods of short bursts of very high frequency waves called 'sleep spindles'. The third and fourth stages are typically combined, and together represent slow-wave sleep.

"This is the type of sleep the body needs to have to feel well rested, because this is when cell repair occurs," said Dr Gilbert.

After about 90 minutes of slow-wave sleep the body goes into its first rapid eye movement (REM) sleep. During this stage, the brain is as active as it is when it is awake. For the rest of the night the body alternates between REM sleep and non-REM sleep.

The American study, which looked at only one type of performance task — visual processing — found that subjects undertaking the same visual task several times throughout the day experienced 'burnout'.

When given a different visual task, the subjects showed the same level of improvement as if they had had a sleep.

'Burnout', or 'switching off' the part of the brain involved in a repetitive task, might be the way the brain prevents losing information that is yet to be stored in long-term memory, the researchers suggested.

Motor learning

The second study, published in this fortnight's issue of Neuron, looked at the role of sleep when learning a motor skill. The researchers found a strong relationship between stage 2 sleep, which previously had not been considered to be very important, and motor learning.

The 62 subjects were given a motor task to learn and then had their sleep measured. The number of 'spindles' in the brain, an identifier of stage 2 sleep, increased during motor learning.

The US researchers think the stage 2 sleep experienced in the last two hours of sleep is the most important, and that it is detrimental to motor learning if you wake up early and lose that sleep.

Dr Gilbert agreed that motor skills seem to be benefited by more stage 2 sleep, but again, he urges caution.

"They have been speculating that more spindles equate to better motor activity because that is all they have measured," he pointed out.

"Simply because two variables are correlated with each other doesn't mean that a change in one causes a change in the other."

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