LAUSANNE, Switzerland — Sleep may seem like a restful period for the mind and body, but a recent study finds our brains actually go into overdrive during this time. Scientists say the gray matter in the brain is much more active as we snooze than previously believed.
An international team carried out magnetic resonance imaging (MRI) scans of 30 people as they fell asleep. After checking, analyzing, and comparing all the data, the findings came as a shock.
“We calculated exactly how many times networks made up of different parts of the brain became active during each stage of sleep,” says study lead author Anjali Tarun, a doctoral assistant at the Swiss Federal Institute of Technology, in a media release.
“We discovered that during light stages of sleep – that is, between when you fall asleep and when you enter a state of deep sleep – overall brain activity decreases. But communication among different parts of the brain becomes much more dynamic. We think that’s due to the instability of brain states during this phase.”
A ‘paradox’ in the sleeping brain
Scientists have long believed brain activity drops as people sleep, but the picture appears to be much more complex.
“What really surprised us in all this was the resulting paradox,” adds Professor Dimitri Van De Ville, head of the Medical Image Processing Laboratory at the École Polytechnique Fédérale de Lausanne (EPFL).
“During the transition phase from light to deep sleep, local brain activity increased and mutual interaction decreased. This indicates the inability of brain networks to synchronize.”
Consciousness has a long connection with the brain’s neural networks, which scientists believe have a link to our introspection processes, episodic memory, and spontaneous thought.
“We saw that the network between the anterior and posterior regions broke down, and this became increasingly pronounced with increasing sleep depth. A similar breakdown in neural networks was also observed in the cerebellum, which is typically associated with motor control,” Prof. Van De Ville reports.
For now, the researchers don’t know exactly why this happens, but the findings are a first step toward a better understanding of our state of consciousness while we sleep.
“Our findings show that consciousness is the result of interactions between different brain regions, and not in localized brain activity,” Tarun explains. “By studying how our state of consciousness is altered during different stages of sleep, and what that means in terms of brain network activity, we can better understand and account for the wide range of brain functions that characterize us as human beings.”
Mapping the sleeping brain in just 2 hours
To probe brain activity while people sleep, study authors turned to electroencephalography (EEG), a method that entails measuring brain activity through electrodes placed along a patient’s scalp.
“MRI scans measure neural activity by detecting the hemodynamic response of structures throughout the brain, thereby providing important information in addition to EEGs,” Prof. Van De Ville says.
During the experiments, Tarun relied upon EEG to identify when the study participants had fallen asleep and to pinpoint the different stages of sleep. She then examined the MRI images to generate spatial maps of neural activity and determine different brain states.
Working with Professor Sophie Schwartz at the University of Geneva and Professor Nikolai Axmacher at the Ruhr-University Bochum, Germany, she could leverage simultaneous MRI and EEG data from the participants. The brain-activity data covered a period of nearly two hours as they were sleeping in an MRI machine.
“Two hours is a relatively long time, meaning we were able to obtain a set of rare, reliable data,” Tarun concludes. “MRIs carried out while a patient is performing a cognitive task usually last around 10–30 minutes.”
The findings appear in the journal iScience.
SWNS writer Mark Waghorn contributed to this report.