Sleep’s final stage key to development

A recent study of the role of rapid eye movement (REM) sleep in the development of young brains suggests that it makes experiences “stick” in the brain. The discovery was published in Science Advances by Professor of Medicine Marcos Frank and his former graduate student Michelle Dumoulin Bridi.

Frank said their findings emphasize the importance of REM sleep in early life and point to a need for caution in giving young children REM-suppressing medications like antidepressants and stimulants for ADHD.

The idea for Frank’s study came from earlier research that suggested a relationship between sleep and developmental brain plasticity—the brain’s ability to reorganize itself by forming new connections between neurons, or brain cells. He said this form of plasticity was first described in the visual system through studies that showed that there is a critical period in a young animal’s development in which vision shapes the connections in the cortex, the brain’s main processing area. These studies also showed that sleep was required for those changes to fully express themselves, but didn’t look at the role of the different stages of sleep. Frank specifically set out to study the role of REM sleep, the final of five sleep stages that make up a sleep cycle. REM sleep is the stage during which we show fast, random eye movements and experience vivid dreams. Inadequate REM sleep has been associated with chronic insomnia and mood disorders and impedes memory and learning.

“It had been theorized that REM sleep is important for brain development, because infants spend up to 70 percent of their sleep time in REM sleep, as compared to adult humans, who typically only spend about 20 percent of their sleep in the REM stage,” said Frank. “But no one could explain what exactly REM sleep is doing in the developing brain, which made it a particularly interesting state to examine.”

During their study, the researchers had young animals wear a patch over one eye, which caused their brains to start changing neuronal connections in the visual cortex, the part of the brain that processes visual information. They monitored the animals’ brain activity while awake and asleep and randomly assigned them to one of two groups. One group was allowed to sleep normally, whereas animals in the other group were consistently woken up—some during REM sleep and others during non-REM sleep. The researchers found that those that went without REM sleep didn’t show normal brain development. This suggests that REM sleep is required to solidify changes in the visual cortex. Furthermore, they found that a brain protein named ERK, which is involved in these types of changes, did not activate in animals that had been deprived of REM sleep.

Frank and Dumoulin Bridi also looked at brain activity patterns during REM sleep and found they are very similar to those seen during wakefulness.

“It appeared as though animals’ waking experiences were being reactivated while they slept,” said Frank. He said further studies are needed to fully uncover the biochemical process by which REM sleep fixes brain changes, which may eventually lead to new therapies to treat brain injury and neurodegenerative diseases.