But up until now, direct evidence has been lacking on how exactly sleep strengthens the brains’ neural connections.
Now researchers in New York have, for the first time, provided clear physical evidence that sleep fortifies learning.
The finding adds to research that a lack of shut eye causes rogue proteins to build up in the eye, increasing the risk for Alzheimer’s disease.
Using a microscope, scientists looked inside the brains of mice to see what happened when they were either asleep, or sleep-deprived, after being trained to walk on top of a rotating rod for the first time.
They found learning led to the formation of new dendritic spines – tiny structures that project from the end of nerve cells and help pass electric signals from one neuron to another – but only in the mice left to sleep.
The study, published in Science, provides the first physical evidence of how sleep helps to consolidate and strengthen new memories.
It revealed learning and sleep cause changes in the motor cortex area of the brain, a region responsible for voluntary movements.
Professor Wen-Biao Gan, of New York University, said: ‘We have known for a long time sleep plays an important role in learning and memory. If you do not sleep well you will not learn well.
‘Here we have shown how sleep helps neurons form very specific connections on dendritic branches that may facilitate long-term memory.
‘We also show how different types of learning form synapses on different branches of the same neurons, suggesting that learning causes very specific structural changes in the brain.’
On the cellular level, sleep is anything but restful.
Brain cells that spark as we digest new information during waking hours replay during deep sleep, also known as slow wave sleep.
This is when brain waves slow down and rapid-eye movement, as well as dreaming, stops.
Scientists have long believed this nocturnal replay helps us form and recall new memories, yet the structural changes underpinning this process have remained poorly understood.
So Dr Gan and colleagues used mice genetically engineered to express a fluorescent protein in neurons to find out exactly what is going on.
A laser scanning microscope illuminated the glowing fluorescent proteins in the motor cortex, allowing the scientists to track and image the growth of dendritic spines before and after the mice learned to balance on a spin rod.
Dr Gan said: ‘It is like learning to ride a bike. Once you learn it, you never forget.’
The researchers trained one set of mice to sleep for seven hours and another to stay awake for the same period of time, after both groups practised for 60 minutes.
The mice lacking in sleep experienced significantly less dendritic spine growth than the well rested ones.
Furthermore, the type of task learned determined which dendritic branches spines would grow.
Running forward on the spinning rod, for instance, produced spine growth on different dendritic branches than running backward on it, suggesting learning specific tasks causes specific structural changes in the brain.
Dr Gan said: ‘Now we know when we learn something new, a neuron will grow new connections on a specific branch.
‘Imagine a tree that grows leaves on one branch but not another branch. When we learn something new, it is like we are sprouting leaves on a specific branch.’
Finally, the scientists showed that brain cells in the motor cortex that activate when mice learn a task reactivate during slow-wave deep sleep. Disrupting this process, they found, prevents dendritic spine growth.
Their findings offer an important insight into the functional role of neuronal replay, the process by which the sleeping brain rehearses tasks learned during the day observed in the motor cortex.
Dr Gan: ‘Our data suggest neuronal reactivation during sleep is quite important for growing specific connections within the motor cortex.’