Sleep is the time when the organism picks up, muscles grow, memories form and ... fat gets stored...And what's even more important, when brain regains its capacity.

A new research has proved that sleep deprivation can severely decrease the brain's ability to store information. People deprived of sleep stored roughly 10% less information than well-rested subjects, "a worrying finding" as the sleep quality is decreasing sharply.

A team at Harvard Medical School in Boston, Massachusetts, US, asked 14 subjects to stay awake one night by playing board games and checking email in the lab. The next evening, they watched 150 images, while their brains were scanned, before going home to sleep.

After two good nights' rest, the subjects passed through another test: they had to identify the 150 images they had seen before from a series of 225 pictures. They were able to choose about 74% correct pictures, on average.

In a comparison group, who had slept well, the identification was 86 % correct.

The fMRI scanner images of the brain showed that sleep deprivation decreases activity in the hippocampus, a brain zone linked to memory processing. "This study shows that the brain has to be well rested to receive and store information for memory processing," says Seung-Schik Yoo, on of the researchers.

Researches made on rats had revealed that sleep deprivation can rise levels of stress hormone in the brain, disturbing the nerve activity in the hippocampus. Researchers believe that a similar mechanism would causes memory impairment in sleep-deprived humans. Previous researches pointed out that a full night's rest after studying increases the learning ability.

Yoo points to the fact that the team's new research is the first to prove the importance of sleeping in neural activity and stocking new information. "How many hours of sleep is good enough? We don't have the answer to that yet," he adds.