Sleep strengthens some synapses

While the body snoozes, the brain is hard at work. Connections between certain nerve cells appear to strengthen during slumber, a study in mice suggests. The results, published in the June 6 Science, may help explain how sleep cements information in the brain.

The new findings emphasize the importance of sleep for learning and memory, says study coauthor Wen-Biao Gan of New York University School of Medicine. “When you go to sleep, you’re not really wasting your time,” Gan says. “You’re actually making connections better.”

Previous studies have found that post-learning snoozes make brains sharper, but just how those improvements happened was unclear. 

Gan and his colleagues studied specific nerve cells, or neurons, that help mice perform a newly learned balancing trick. Over an hour, mice learned to run on a rod as it rotated faster and faster. Neurons in the brain’s motor cortex helped the mouse stay on the rod as it spun more rapidly.

Right after their training session, some mice were allowed to sleep for seven hours. Neurons in these mice’s motor cortices sprouted new dendritic spines, which are docking sites where other neurons can connect, Gan and his team found. But mice that were kept awake didn’t form as many new spines. These sleep-deprived mice were also worse at balancing on the rod days later, the team found. These new docking sites may not all have become working synapses — the communication conduits between neurons. But the presence of the new dendritic spines suggests that overall, connections between the neurons are getting stronger, the researchers say.

In separate experiments, the researchers found that motor cortex neurons that were active when mice ran on a treadmill were also active later during non-REM sleep, which spans several stages of sleep and accounts for about three-quarters of sleep time in people. This neural rehashing during sleep might be the signal for neurons to form new connections, Gan proposes.

Discovering that these neurons’ connections get stronger during slumber is “going to challenge our thinking about what sleep is doing in the brain,” says neuroscientist Marcos Frank of the University of Pennsylvania, in Philadelphia. “It really provides incontrovertible evidence that sleep can promote the formation of synapses.”

And that evidence flies in the face of a popular idea about what sleep does to the brain, Frank says. That idea, called the synaptic homeostasis hypothesis, proposes that synapses weaken during sleep, a cleanup that clears the brain of clutter and prepares it for a new day of learning (SN Online: 6/23/11). The ideas aren’t necessarily at odds, though: Some synapses — those involved in learning a new trick, for instance — may get strengthened, while others weaken during sleep.

Synaptic strengthening might be specific to neurons involved in movement, says Frank. “This is one kind of neuron in one part of the brain,” he says. But similar synapse strengthening during sleep might also happen elsewhere. He and his colleagues have evidence that some neuron connections involved in vision also get stronger during sleep.