Study supports theory that sleep is important for learning, memory

(Republished with permission from the St. Louis Post-Dispatch. This article originally ran in the Science & Medicine section on Thursday, Sept. 21, 2006.)

By Tina Hesman Saey

Sleeping fruit flies are pictured through a microscope for a sleep deprivation study.
Sleeping fruit flies are pictured through a microscope for a sleep deprivation study.

If you want your children to be smarter, send them for play dates followed by good long naps, suggests a new study by researchers at Washington University and the Neurosciences Institute in La Jolla, Calif.

The study shows that fruit flies need longer naps to help them learn social lessons, and the need for sleep is tied to some memory genes. Humans have those genes.

Researchers can’t tell if the flies dream at night as humans do. But they take midday naps. They sleep at night. And they respond to caffeine and antihistamines the way people do. Young flies sleep a lot, much the way human infants do.

Scientists have known that people and animals need sleep, but they didn’t know why. This study rejuvenates and advances an old idea that sleep is important for learning and memory. That idea had fallen out of favor with many sleep scientists, who thought you needed sleep to clear the body of certain chemicals, said Indrani Ganguly-Fitzgerald, a researcher at the Neurosciences Institute. She is the lead author of the new study, which appears today in the journal Science.

Ganguly-Fitzgerald and her colleagues Paul J. Shaw and Jeff Donlea at Washington University experimented with fruit flies, known to scientists as Drosophila, because they share many genes with humans.

The researchers placed some flies in vials with up to 40 others. Other flies got their very own vials. Flies who socialized took longer naps than their solo counterparts.

Blind flies and those with no sense of smell didn’t increase their napping time after social sessions. Those flies don’t have as much information to process during their siestas, so they don’t need extra sleep.

Donlea schooled male flies about rejection to show that sleep was important for learning social skills. He placed a virgin male fly in a vial with a female who had already mated and wasn’t up for further action.

The hapless male performed a mating dance and rubbed his wings together to serenade his potential mate, Shaw said. After repeatedly crashing and burning, the male gave up.

The researchers allowed some flies to sleep after the session, and kept others awake by nudging them whenever they settled down to nap. Two days later, the researchers put the males in vials with virgin female flies. Only the flies who napped after making failed passes learned that the females were just not that into them. Sleep-deprived flies made moves on the new females as if they’d never been shot down.

The researchers tested 43 different strains of flies with mutations in genes that help make long-term or short-term memories. Only 17 of the genes were linked to the need to sleep more after socializing. Some of those genes are active in the mushroom bodies of the fly’s brain, a structure similar to the hippocampus in humans. That brain area is involved in making long-term memories.

The study demonstrates a link between what an animal experiences during waking hours with what happens in the brain during sleep, said Marcos Frank, a sleep researcher at the University of Pennsylvania.

Other researchers have attempted studies of sleep and memory in rats and mice and humans, but scientists can’t use the same genetic methods to dissect the brains of people and rodents they can with fruit flies to see how genes and brain cell circuits change during sleep, he said.

“The black box stays closed in those (rodent and human) studies. This kind of study is what you need to open that black box up,” Frank said. “I’ve never seen results as clear as this in a rodent model.”

Until the researchers can demonstrate how brain circuits change during sleep, some critics will probably still declare sleep a side-effect of being awake, rather than a process necessary to learn and form memories, Frank said. He cautioned against stretching the results too far to imply insect sleep is identical to human sleep but said that plenty of good studies show that napping can help people learn complex tasks.

Copyright 2006 St. Louis Post-Dispatch, Inc.