Study reveals how sleep boosts learning

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Two distinct sleep stages appear to play vital, complementary roles in learning: one stage enhances overall performance, while the other stabilizes what we learned the previous day.

Scientists have long known that a good night’s sleep works wonders for our ability to learn new skills.

What has been less clear is the role of different sleep stages. In particular, there has been controversy over the relative contributions of rapid eye movement (REM) sleep, which is when most dreaming occurs, and non-REM sleep, which is mostly dreamless.

Their experiment — which focuses on visual learning — suggests that rather than one stage being more important than the other for learning new skills, both play essential and complementary neurochemical processing roles.

They found that while non-REM sleep enhances our performance of newly acquired skills by restoring flexibility, REM sleep stabilizes those improvements, and prevents them from being overwritten by subsequent learning.
“I hope this helps people realize that both non-REM sleep and REM sleep are important for learning,” says corresponding author Yuka Sasaki, a professor of Cognitive, Linguistic, and Psychological Sciences at Brown.

Most REM sleep occurs in the final hours of sleep, so the finding reinforces the importance of not cutting short these later stages.
“When people sleep at night, there are many sleep cycles. REM sleep appears at least three, four, five times, and especially in the later part of the night. We want to have lots of REM sleep to help us remember more robustly, so we shouldn’t shorten our sleep.”

Psychologists have previously identified two distinct benefits of sleep for learning.

The first benefit, which they call “offline performance gains,” means the learning acquired before sleep is enhanced after sleep, without any additional training.

The second benefit, called “resilience to interference,” protects the skills learned before sleep from being disrupted or overwritten by subsequent learning after awaking.

To reap both benefits, there is a trade-off between flexibility and stability.

Learning during the day involves forming new synapses, which are the electrical connections between nerve cells, and the strengthening of existing synapses through repeated use.

While we sleep, the brain appears to streamline its operations to work more efficiently. According to a leading hypothesis, it does this by reactivating synapses that have been strengthened during the day, and then indiscriminately ‘downscales’ or weakens them all.
This restores flexibility, or plasticity, to the brain’s local connections and wider networks, to improve overall performance.
At the same time, during sleep, the brain must also stabilize key synapses to prevent what was learned the previous day from being eliminated by new learning experiences.

To investigate when each of these processes occurs during sleep, the scientists gave volunteers a standard visual learning task. This involved identifying letters and the orientation of lines that pop up on a screen in two different tasks: one before sleep and one after sleep.

The letters and lines were displayed against a fixed background of horizontal lines for one group of volunteers, and vertical lines for another group.

Participants were then allowed to sleep for 90 minutes with their heads inside an MRI scanner.

After awakening, they were given 30 minutes to fully wake up before performing the same task, but with the opposite orientation of background lines.

Previous research has shown that switching the orientation of background lines interferes with performance gains on this learning task.

A third group of volunteers was not given any learning task before or after sleep.

The researchers used electrodes glued to subjects’ eyelids and scalps to detect when they entered different sleep stages.