Scientists who study sleep are ready for an awakening.
After decades of research, and many unproven theories, researchers still haven't figured out what sleep is good for. But some think they can learn why people sleep by studying an animal that can't even shut its eyes.
Recent research has suggested that fruit flies, the tiny pests that hover above overripe bananas, need their rest just like people do. New studies being reported this week go even further and hint that fruit flies need to "sleep" in order to remember what they learned during the day. Scientists hope that these and other fruit fly studies under way will help explain why people spend a third of their lives getting shut-eye.
"There is no comparable biological function which is so universal and so time consuming for which we know nothing about," said Giulio Tononi, of the Neurosciences Institute in San Diego. "It's a mystery which we are not able to crack."
It's not for a lack of theories, Dr. Tononi said. Scientists have suggested that animals sleep to conserve energy, to replenish the stores of energy in the brain, or simply to stay out of trouble.
None of these ideas have held up. But researchers think they know sleep when they see it.
To qualify as a sleeper, an animal needs to do three things: Keep still for extended periods, ignore outside commotion, and rest longer if a pesky scientist has been keeping it awake.
Fruit flies qualify. Dr. Tononi and his colleagues reported recently in the journal Science that the tiny, red-eyed insects do all these things. Researchers at the University of Pennsylvania reported similar results earlier in the journal Neuron.
Technically, the scientists don't know for sure if the fruit flies are truly sleeping like people do, because they don't know why people sleep.
"My personal opinion is we will not have the definitive answer until we have the function of sleep," Dr. Tononi said. "Until we have that, the best we can do is build the case."
But so far, it seems like flies behave a lot like the people who like to swat them.
Flies rest for almost eight hours a day. Younger flies rest more than older ones. Give the flies caffeine, they rest less. Give them an antihistamine, they snooze sooner. The genes that kick on in a resting fly are the same genes that kick on in a sleeping rat, and presumably, a sleeping scientist.
Although they don't put their heads down on a pillow, flies may even have a peculiar "sleeping" position, crouching slightly as they rest, said Joan Hendricks, a sleep scientist on the Pennsylvania research team. And flies that are resting even seem to try to avoid late-night partying flies, Dr. Hendricks said.
Fruit fly video surveillance has caught it on tape.
"Every so often, a partyer would come over and try to get a rise out of the ones that were resting" on the other side of the lab dish, she said. "The resting ones don't respond at all."
Dr. Hendricks said she suspects the flies are resting to fine-tune their brains after a hard day of searching for rotten fruit.
Scientists discovered years ago that fruit flies can actually remember things, and have even found genes that help the flies make memories. In recent experiments, Dr. Hendricks has found that blocking one of those memory genes makes for especially sleepy flies. She suspects that one purpose of resting is to help the memory gene work. When the gene is blocked, she said, the flies may be resting extra long to try to unblock the gene.
More work needs to be done to nail down the theory, Dr. Hendricks said. She will report her new experiments Tuesday at a meeting of the Associated Professional Sleep Societies in Las Vegas.
Even if her theory holds true, it doesn't mean that sleep evolved only to promote learning, she said. During evolution, animals may have slept to keep out of harm's way, and then other functions â€“ like learning â€“ got piggybacked on to sleep, she said.
Some scientists are still skeptical about whether the flies are really sleeping in the way that people and other mammals do.
"So far, they don't know if the rest phenomenon originates in the brain, or if the brain is truly required," said Masashi Yanagisawa, a biologist at the University of Texas Southwestern Medical Center at Dallas. "In my opinion, that's important to show that this rest is equivalent to mammalian sleep."
Mammals, including people, have distinct brain wave patterns when they sleep, the main evidence that people's sleep originates in the brain.
But studying fruit flies to understand people has a good track record, scientists say. And if flies can help people understand sleep, the studies are time well spent, said Irena Tobler, a sleep researcher at the University of Zurich in Switzerland.
"Think in terms of your own life," Dr. Tobler said. "If you reach 60, you spent 20 years sleeping. This is not peanuts."