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Although it is well known that sleep plays an important role in memory, what actually happens inside the brain was unknown.
Professor Wen-Biao Gan of New York University, told the BBC: "Finding out sleep promotes new connections between neurons is new; nobody knew this before.
"We thought sleep helped, but it could have been other causes, and we show it really helps to make connections and that in sleep the brain is not quiet, it is replaying what happened during the day and it seems quite important for making the connections."
The researchers trained mice to walk on top of a rotating rod - a previously unknown skill. They then looked inside their living brains to see what happened during sleep or sleep deprivation.
Their findings, published in the journal Science, showed that sleeping mice formed significantly more new connections between neurons, proving that they were learning more. Deep sleep, which is when the brain replays the day's activity, was shown to be necessary for memory formation.
With sleep being proven again and again to be vital to our psychological and physical health, are you worried you're not getting enough?
In 2002 American researchers analysed data from more than one million people, and found that getting less than six hours' sleep a night was associated with an early demise - as was getting over eight hours.
High blood pressure is more than three times greater among those who sleep for less than six hours a night, and women who have less than four hours of sleep are twice as likely to die from heart disease. Other work suggests that a lack of sleep is also related to the onset of diabetes, obesity, and cancer.
Light towards the blue end of the spectrum - used in computer screens, smartphones, flat-screen televisions, and LED lighting - suppresses the production of the sleep-inducing hormone melatonin. Using these before bed, which an increasing number of people do, may stop us from dozing off to sleep.
get them from a bike shop and wear using computer after sunset.