0422 GMT January 19, 2022
In magnetic resonance imaging brain scans of sleeping study participants, the researchers found that brain network activity peaks during non-rapid eye movement stage 2, or as people fall asleep, and decreases during stage 3, or deep sleep, UPI reported.
Interactions between brain networks also break down with increasing sleep depth, the researchers said.
"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," study coauthor Dimitri Van De Ville said in a statement.
"We think that's due to the instability of brain states during this phase," said Van De Ville, a professor of radiology at the University of Geneva in Switzerland.
Research suggests that a person's state of consciousness changes significantly during stages of deep sleep, just as it does in a coma or under general anesthesia, according to the researchers.
Given that, scientists have long believed that brain activity declines during sleep, although this has yet to be proven in research, they said.
Non-rapid eye movement sleep has three stages. Stage 1 occurs as a person falls asleep and usually lasts fewer than 10 minutes. Stage 2 lasts from about 30 to 60 minutes and is characterized by muscle relaxation and slowed brain activity.
Stage 3 is deep sleep and lasts about 20 to 40 minutes before the onset of rapid eye movement sleep, the deepest level of sleep when most dreaming occurs, Michigan Medicine said.
Historically, most research on sleep has been conducted by using electroencephalography, which entails measuring brain activity through electrodes placed along a patient's scalp, the researchers said.
However, MRI scans can measure neural activity by detecting the blood-flow response of structures throughout the brain, they said.
For this study, the researchers used EEG to identify when the 30 study participants had fallen asleep and to pinpoint the different stages of sleep. They then examined MRI images of participants' brains during sleep to generate spatial maps of neural activity and determine different brain states.
Despite the inherent challenges in performing MRIs on participants while they were sleeping, due to noise made by the machines, the researchers were able to obtain brain-activity data covering a period of nearly two hours.
MRIs performed while a patient is performing a cognitive task usually last 10 to 30 minutes, the researchers said.
Based on the scans, consciousness is the result of interactions between different brain regions, and not in localized brain activity, and many of these interactions start to break down during deep sleep.
These brain networks may be linked to thought processes, as well as episodic memory and spontaneous thought, the researchers said.
A similar breakdown in brain networks was also observed in the cerebellum, which is typically associated with motor control, they said.
"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," study coauthor Anjali Tarun, a doctoral assistant at the Swiss Federal Institute of Technology Lausanne, said i