Life

Sounds played as you sleep can reinforce memories, suggest Ken Paller and his colleagues at Northwestern University in Evanston, Illinois.

They asked people to memorise which images and their associated sounds – such as a picture of a cat and a miaow – were associated with a certain area on a computer screen and then to take a nap. They played half the group the sounds in their sleep, and these people were better at remembering the associations than the rest when they woke up.

Paller hopes sounds can be used to improve all kinds of memory and next he'll be figuring out if we can learn languages while we snooze. But before you nod off, New Scientist helps you get the most out of your shut-eye.

How can you boost your sleep learning capacity?

As a rule, hit the hay after learning something new – late-night TV and Xbox marathons are a no-no.

That is, of course, unless the skill you hope to learn is a computer game: when Sidarta Ribeiro of the Edmond and Lily Safra International Institute of Neuroscience in Natal, Brazil, got people to play shoot-'em-up video game Doom before bed, those who dreamed about the game during their sleep were better players the next day.

Once asleep, playing sound cues (see above) may work for some, but if you like to slumber in silence, try smells instead. A couple of years ago, Björn Rasch and his colleagues at the University of Lübeck in Germany found that people were better at remembering where objects belonged on a computer screen when they were reminded with the scent of a rose, which they had smelled during the learning task, and again during their sleep.

What's better – one long sleep or lots of short naps?

Take a leaf out of granny's book – afternoon naps are good for you. Jim Horne, who researches sleep at Loughborough University, UK, thinks that even a 10-minute kip can improve performance, and that this could be lifesaving in the case of overtired drivers and nurses. Such short shut-eye means that you won't fall into a deep sleep, so you can easily recover without experiencing "sleep inertia" (see below).

In fact, others claim that it is the process of drifting off, rather than the deep sleep itself, that is good for you. Olaf Lahl's students at the University of Düsseldorf, Germany, performed better in memory tests only 5 minutes after falling asleep.

However, don't give up on long sleep: rapid-eye movement (REM) sleep, which occurs later in sleep, is thought to be important for memory processing.

What's this about people who don't sleep enough getting fat?

Our hectic modern lifestyles and shorter visits to the land of nod have been linked by some to expanding waistlines. Sanjay Patel (PDF) and his colleagues at Case Western Reserve University in Cleveland, Ohio, put the theory to the test in 2006 and found that short-sleeping women were 15 per cent more likely to become obese than their well-rested counterparts.

Jim Horne, on the other hand, reckons such effects might be overblown. In Patel's study, the average difference in weight gain between the two groups was a measly 700 grams over 10 years. Besides, Horne says, we're probably getting about the same amount of sleep as we ever did – around seven and a quarter hours.

Why does lack of sleep make you grumpy and groggy?

Even the best of us has woken up confused and disorientated, usually when we're awoken during a deep sleep, known as "slow-wave sleep". David Dinges of the University of Pennsylvania in Philadelphia found that people who were woken up during this deep sleep couldn't do simple arithmetic, a phenomenon called "sleep inertia".

But the grumpiness associated with a lack of sleep could be due to a different point in the sleep cycle, REM sleep. This is when we do most of our dreaming, and some think dreams are important for processing the emotions we experience during the day. Matt Walker, a psychologist at the University of Berkeley, California, has even described this process as "overnight therapy".

How does sleeping improve your memory?

Deszo Nemeth, a psychologist at the University of Szeged in Hungary, suggests that while we sleep, short-term "working memories" are transferred from the hippocampus of the brain to the cortex, where they become more stable, long-term memories.

Meanwhile, Catherine Siengsukon of the University of Kansas Medical Center in Kansas City reviewed the evidence for "offline practice" – practising skills during sleep – earlier this year. She reckons that motor learning – training brain areas that control muscles - during sleep could help rehabilitate young, brain-damaged patients.

When does sleep learning take place?

Different stages in the sleep cycle are important for different types of memory. REM sleep seems to be important for perceptual memory, "like when you're learning to play darts", says Paller, while the consolidation of "declarative" memories – facts and events – happens during deep slow-wave sleep.

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