College students probably won’t heed the advice, but new research confirms that sleeping is essential for learning and remembering.
Sleep’s function has long been a mystery (SN: 10/24/09, p. 16), but many researchers have gathered evidence that it is important for learning and memory. Two new studies confirm that sleep plays a central role in solidifying memories and preparing the brain for new learning.
Tickling a few neurons located at the top of the fruit fly brain triggers the insects to sleep, researchers led by Paul Shaw at Washington University in St. Louis discovered. Turning on the sleep-initiating brain cells makes short-term memories into long-lived ones, the researchers report June 24 in Science. A separate study in the same issue of Science, by researchers at the University of Wisconsin–Madison, describes microscopic evidence that during sleep, connections between brain cells are pruned. The group had indirect evidence that sleep prepares the brain for learning the next day through pruning, but the new study presents direct confirmation.
The findings “confirm that learning and memory were important way back in the evolution of sleep,” says Marcos Frank, a neuroscientist at the University of Pennsylvania. He was not involved in either study.
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Shaw’s group genetically engineered fruit flies with what amounts to a remote-control sandman. The team modified flies to make a protein that switches the sleep-inducing neurons on in response to heat, including both hot temperatures and the burn of capsaicin, the chemical that gives chilies their spice. Engineered flies that ate capsaicin nodded off, but not right away. Instead, putting the fruit flies in an incubator set for 31°Celsius (about 88° Fahrenheit) induced immediate Z’s.
Engineered male fruit flies were introduced to other male flies that had been doused with female pheromones. The males did their best to court the pseudo-females, only to get rejected again and again for hours. But, like staying up all night cramming for an exam, the experience didn’t lead to long-term memories if the flies didn’t sleep right after a futile wooing session. Put into a cage two days later with faux females, the same engineered flies seemed to have no recollection of their previous rejection marathon.
But when the researchers flipped the sleep switch right after the would-be Romeos had finished their ill-fated courtship, the normally brief memory of the rejection turned into a lasting one. Stimulating other parts of the brain did not turn short-term memories into long-term ones. And flies forced to stay awake in the heat also didn’t remember the rejection, indicating that sleep, not just brain stimulation, was responsible for turning short-term memories into long-term ones.
“Ultimately, what we’re saying is that sleep is playing a productive role — it’s not a passive role — in memory consolidation,” says Shaw, although it is not clear how sleep accomplishes the task.
One way sleep is thought to affect memory is by getting rid of extraneous memories that take up valuable resources. This idea, called the synaptic homeostasis model, holds that during sleep, connections, or synapses, between brain cells are weakened. Connections that started out weak may be severed, but strong ones might survive. That enables a brain that is full of a day’s experiences to reset for the next day. Shaw’s group shows in the new study that sleeping and synapse pruning go hand-in-hand.
In a separate study, researchers at the University of Wisconsin–Madison show the pruning in microscopic detail. The more you learn, the more synapses are formed and the more sleep is needed to get the brain back to its fresh state, the team found in experiments with fruit flies at the “fly mall.”
The fly mall is nothing fancy, says Chiara Cirelli, a neuroscientist and study coauthor. It’s really just a liter bottle where flies can fly and mingle. That may not sound like much, but for fruit flies that have spent the first week of their lives in solitary confinement in tiny tubes that don’t allow them to stretch their wings, a visit to the fly mall is a big deal.
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In three parts of the fruit fly brain, more synapses were found at the end of the big day than at the beginning. Flies that slept pruned the number of connections back to the morning’s levels, but sleep-deprived flies did not, the team found.
The researchers don’t know which synapses are severed or how the process happens. Cirelli says the team is conducting experiments with mice to try to answer those questions.
But the important thing to remember is that sleep is crucial for making memories, Frank says.
“It’s not only that sleep makes your brain more able to learn the next day by this kind of pruning,” he says, “but if you don’t have sleep, you can’t form the memory at all.”