New research manipulates the deep sleep phase and examines its effects on learning new skills.
A team of researchers from the University of Zurich (UZH) and the Swiss Federal Institute of Technology (ETH) in Zurich, both in Switzerland, set out to examine the effect of a disturbed deep sleep phase on the brain's ability to learn new things.
More specifically, the new study - published in the journal Nature Communications - looks at the brain's ability to change and adapt in response to the stimuli that it receives from the environment, or neuroplasticity, in the motor cortex and how it is affected by deep sleep.
The motor cortex is the brain area responsible for developing and controlling motor skills, and the deep sleep phase - also called slow-wave sleep - is key for memory formation and processing, as well as for helping the brain to restore itself after a day of activity.
Manipulating the motor cortex during deep sleep
The study involved six women and seven men who were asked to perform motoric tasks during the day following a night of unperturbed sleep, and after a night during which their deep sleep had been disturbed.
The tasks involved learning a series of finger movements, and the researchers were able to locate precisely the brain area responsible for learning movement.
Using an electroencephalogram, the researchers monitored the brain activity of the participants while they were sleeping.
On the first day of the experiment - after the first movement learning session - the participants were able to sleep without disturbance.
On the second night, however, the researchers manipulated the participants' sleep quality. They were able to focus on the motor cortex and disrupt their deep sleep, thus investigating the impact that poor sleep has on the neuroplasticity involved in practicing new movements.
The participants did not know that their deep sleep phase had been tampered with. To them, the quality of their sleep was roughly the same on both occasions.
Poor sleep keeps synapses excited, blocks the brain's ability to learn
Next, the researchers evaluated the participants' ability to learn new movements. In the morning, the subjects' learning performance was at its highest, as expected.
However, as the day progressed, they continued to make more and more mistakes. Again, this was expected.
After a night of restorative sleep, the participants' learning efficiency spiked again. But after their night of manipulated sleep, their learning efficiency did not improve as significantly. In fact, the morning after a night of manipulated sleep, the participants' performance was as low as on the evening of the previous day.
The reason why this happens, according to the researchers, is that during the manipulated deep sleep, the neurons' synapses did not "rest" as they normally would during restorative sleep.
During the day, our synapses get excited as a response to the stimuli that surround us. During sleep, however, these synapses restore themselves and their activity "normalizes." Without this restorative period, the synapses stay maximally excited for too long. Such a state inhibits neuroplasticity, which means that learning new things is no longer possible.
"In the strongly excited region of the brain, learning efficiency was saturated and could no longer be changed, which inhibited the learning of motor skills," explains co-lead author Nicole Wenderoth, professor in the Department of Health Sciences and Technology at the ETH Zurich.
To ensure that they located the right brain area responsible for deep sleep, the researchers repeated the experiment by assigning the same task but manipulating a different region of the brain.
This did not result in any changes to the participants' performance.
This is the first time that a study has proven the causal connection between deep sleep and learning efficiency.
Reto Huber, professor at the University Children's Hospital Zurich and of child and adolescent psychiatry at UZH, comments on the significance of the study:
"We have developed a method that lets us reduce the sleep depth in a certain part of the brain and therefore prove the causal connection between deep sleep and learning efficiency [...] Many diseases manifest in sleep as well, such as epilepsy. Using the new method, we hope to be able to manipulate those specific brain regions that are directly connected with the disease."