Even Fruit Flies Catch Their Zs
Rats do it, cats do it, birds do it, dolphins and people too. We sleep--an activity so widespread, so important and so basic, it’s usually taken for granted.
That we mammals need sleep is clear. Deprived of sleep, a rat will die in 17 days. One day without sleep, and humans struggle mightily to stay awake. Sleep requires time, and even energy. But biologists have no idea why we need that sleep, or what function it actually serves.
This basic question--why do we sleep?--remains “a huge hole in biological knowledge,” said Allan Rechtschaffen, a pioneering sleep researcher at the University of Chicago who first proved that sleep deprivation is lethal to rats.
Is sleep for rest? To conserve energy? For development? To cool the brain? To warm it? To boost the immune system? To learn and memorize? Or to unlearn, ridding our brains of excess mental baggage? All these ideas have been proposed, leading to suggestions that insomniacs would do well to count sleep theories in lieu of sheep.
“There’s anything but consensus,” said Paul Shaw, a sleep researcher at the Neurosciences Institute in San Diego. “The only consensus is, we don’t know.”
Biologists have been stymied by complex biological processes before. The response of our bodies and brains to daily circadian rhythms is one example. The vaunted ability to learn and remember are others.
Scientists probing the genetics and physiology underlying those activities have been ably assisted by the humble lab denizen Drosophila melanogaster, the tiny fruit fly.
Flies have become a classic genetic tool because they can be quickly bred in prodigious numbers, require little space or care, and, most important, can be screened for mutants--animals with genetic mutations that differ significantly in the aspect being studied. In learning studies, for example, a strain of flies with a genetic mutation called dunce had trouble in learning tests. In memory studies, another strain, labeled amnesiac, proved forgetful.
But first, scientists had to prove to critics that these simple organisms were capable of something as complex as learning. Flies easily learned to avoid an odor associated with a mild shock, and they learned to approach odors linked to a tasty reward.
Such experiments “established the fly as a bona fide learner, if not a genius,” wrote Ralph J. Greenspan, a fly geneticist at the Neurosciences Institute, who recently turned his attention to what his flies might tell researchers about the role of sleep.
Earlier this year, Greenspan and Shaw’s team made a crucial advance. They showed that flies do indeed sleep, nap and get tired when their schedules are disrupted. “They’ll be moving around, doing what flies do--which is eating and having sex--and some of them will settle down and become immobile,” said Shaw, the lead author on a paper describing the finding, which was published in the journal Science.
A similar finding was made by a University of Pennsylvania group led by Joan C. Hendricks. In 1983, Swiss researcher Irene Tobler described a sleep-like state in cockroaches, but whether the ubiquitous lab fly slept had remained an open question.
Of course, the scientific teams had to do more than simply observe flies having quiet time. At the Neurosciences Institute, vials holding flies were attached to an ultrasound machine. The sound wave emitted by the machine was disrupted any time the fly in the vial moved, accurately recording the fly’s activity.
The insects are remarkably similar to people: They sleep at night and nap during the day. Fruit flies take their name from their early-bird habits. Drosophila is Greek for lover of dew. Around noon, they tend to “have a siesta,” said Shaw.
Shaw is confident enough to dub the behavior sleep because the inactive flies do not respond quickly to disruptions from the outside world--in this case to big pulses of sound from a boombox. Flies that are awake respond to all pulses of sound; sleeping flies respond only to the loud ones.
Flies don’t have rapid eye movement--or the “rapid antennal movement” that observers have reported in bees. Rats routinely show rapid whisker and paw movement. As for whether flies dream? “We’ll tell you after we identify the fly Sigmund Freud,” Greenspan quipped.
Shaw found that flies get sleepy when they miss their 40 winks. In 12-hour sessions, Shaw and his volunteers would sit in a room lit by dim red light and tap on the vials of flies that were trying to rest. The flies exhibited a “sleep rebound.” The longer they were deprived of sleep, the harder they were to keep awake--to the point that Shaw sometimes mistook very sleepy flies for dead ones.
Flies, like humans, got drowsy when they were given antihistamines and had trouble sleeping when caffeine was dissolved in their food. “It woke them up,” Shaw said. “They were awake, wondering what to do with themselves, like all insomniacs.” And like those insomniacs, the flies were much sleepier the following day.
The team also found that young flies, like infant babies, needed the most sleep. Geriatric, month-old flies needed much less sleep and often woke up in the middle of the night. The finding will allow a “genetic assault” on the biology of sleep, said Greenspan, who calls the genetic tools available in flies a “trump card.”
“Now that they have this model, they can study sleep in drosophila where a whole variety of genes have already been knocked out,” said Jerry Siegel, the chief of neurobiology research at the Sepulveda Veterans Affairs Hospital and a professor at UCLA who studies the evolution of sleep by examining sleep in a variety of animals, including the dolphin, platypus, echidna and turtle. “I think it’s very important.”
“The question,” he added, “is how close is insect sleep going to be to mammal sleep?”
Humans are not so different from flies: We share about two-thirds of our genes with our winged cousins. And if circadian rhythms, memory and now sleep are any indication, Greenspan said, “Flies are proving that they are more like us than one might think.”
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McFarling can be reached at usha.mcfarling@latimes.com
