"To sleep, perchance to dream," wrote Shakespeare.
But there`s more to it: scientist have now discovered the mechanism by which a good night`s sleep improves learning and memory.
Researchers at New York University School of Medicine and Peking University Shenzhen Graduate School used advanced microscopy to witness new connection between brain cells – synapses – forming during sleep.
Their study, published in the journal Science, showed even intense training could not make up for lost sleep.
Experts said it was an elegant and significant study, which uncovered the mechanisms of memory. It has long been known that sleep plays an important role in memory formation and learning. But what actually happens inside the brain has been an ongoing source of considerable debate.
For the study, mice were engineered to express a glowing fluorescent protein in their brain cells, which could then be studied by the scientists. The team trained the mice in a new skill – walking on top of a rotating rod, and then compared the growth of neuronal connections as they either slept or stayed awake. Observing the living brain with a microscope, it was discovered that sleeping mice formed significantly more new connections between neurons than their sleep derived partners – they were learning more.
And by disrupting specific phases of sleep, the research group showed that the most new neuron connections were formed during deep or slow-wave sleep, making it necessary for memory formation. They also found that the same brain
cells activated during training were reactivated during this slow-wave deep sleep, and the connections that grew changed depending on what skill was learnt.
It was during this stage, the brain was "replaying" the activity from earlier in the day.
When mice learned motor tasks, small protuberances – or "spines" – formed on some of the dendritic branches of specific brain neurons, which represent the physical correlate of a memory. But the neurons grew and retained these spines better when the mice slept after learning the task. Neurons that fired during learning fired again during subsequent slow-wave sleep, allowing the mice to conserve the newly formed spines – and memories.
Professor Wen-Biao Gan, from New York University, said "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."
This is just the latest piece of science to highlight the importance of sleep.
A new reason for sleep was discovered last year when experiments showed the brain used sleep to wash away waste toxins built up during a hard day`s thinking.
Further tests showed how significant sleep was.
Mice doing up to an hour`s training followed by sleep were compared with mice training intensively for three hours but then sleep deprived.
The difference was still stark, with the sleepers performing better and the brain forming more new connections.
Gan added: "One of the implications is for kids studying, if you want to remember something for long periods you need these connections.
"So it is probably better to study and have good sleep rather than keep studying."