The lions sleep tonight

Animals have different patterns of rest. By understanding them, researchers hope they'll figure out why we snooze

We spend a third of our lives doing it. We build special rooms for it, and we agonize over not getting enough of it. Yet despite all the time humans invest in sleep, scientists have still not been able to explain why we need it.

While an array of lab studies show that slumber-deprived people remember less, react more slowly, and even develop higher risks of heart disease and diabetes, the reasons sleep developed in the first place have remained murky - and some even argue that it may not be as useful as we think.

Several scientists are now trying an innovative approach: comparing the snoozing habits of different animals to better understand how sleep evolved over time.

"The basic question is, why would Mother Nature create such a peculiar phenomenon?" says Dr. Patrick McNamara, an associate professor of neurology and psychology at Boston University who is leading the broadest effort to date, a project called the Phylogeny of Sleep.

McNamara and his colleagues have compiled existing data on close to 150 species of mammals, from killer whales to sloths, in a searchable database available to researchers and the public (bu.edu/phylogeny). In the next few months they will add several dozen birds to the database, a three-year effort funded by the National Institute of Mental Health.

The myriad variations in sleep behaviors they uncovered read like a Cole Porter song: Birds do it while they fly, lions do it more than half the time. Dolphins do it in the water, but their young don't even bother. Male horses get the least amount of sleep of any adult animal in the catalogue, about three hours per day, while the South American armadillo clocks a whopping 20 hours per day.

They found that sleep time changed according to environmental influences - whether the animal is a predator or preyed upon, for example, or whether it sleeps in a group or alone. And the more closely related two species were genetically, the more likely they were to sleep similarly. For McNamara, that is a promising sign that studying the evolution of sleep could eventually reveal its function.

Though scientists have spent years investigating the theory that sleep makes us smarter, often using brain volume as a measure of intelligence, McNamara's team found no association between sleep time and brain volume.

The project, and other attempts at comparing sleep among animals, have also raised interesting questions about rapid eye movement (REM) sleep, the stage of the sleep cycle where the body is paralyzed and the mind experiences intense dreams.

Scientists have long suspected that REM sleep affects brain function, especially memory. In 2006, a team of scientists from Indiana State University compared records for dozens of mammals and concluded that those with higher amounts of REM sleep had larger brains.

Yet when McNamara and colleagues analyzed the same data, after weeding out studies where animals had been monitored only for short periods of time, they found no relationship; in fact, the animal with the most REM sleep was the platypus, a small-brained animal with no obvious need for large amounts of memory.

Humans - whom scientists assume, rightly or wrongly, to be smarter than other animals - exhibited only an average amount of REM compared to their fellow mammals.

Conflicts like these, and the fact that data only exist for a tiny fraction of the 5,000 known mammal species, illustrate how much work still remains to be done.

Part of the problem is defining what constitutes sleep. Some researchers contend that the brain of a human varies so much from the brain of a fruit fly, say, that each experiences sleep completely differently. Birds, for example, sleep with one hemisphere of their brains at a time. While some sleep experts say their behavior shows that it is possible to benefit from sleep without involving the entire body, others argue that it may not count as sleep at all.

Jerome Siegel, a University of California at Los Angeles neuroscientist, has raised eyebrows among his colleagues by arguing that sleep, rather than providing specific benefits, is simply a default mode that allows animals to rest and stay out of trouble when they do not have anything else to do. So lions, he says, luxuriate in long c atnaps because they do not need to worry about predators, and the little brown bat only rouses itself from its cave for the few hours that the flies it feeds on are swarming; when the flies retire, so does the bat.

In Siegel's own experiments with dolphins and killer whales, the babies of the species did not sleep at all during their vulnerable first few weeks of life, when they were busy migrating to feeding grounds and avoiding predators - with no obvious impact on their development.

"Humans in the 21st century are constantly thinking that if we could only stay awake longer, we would be so much more successful and happy," Siegel says. "But in point of fact, for most animals the most adaptive thing they can do is be asleep because it's a way of saving energy and being safe."

In this view, REM sleep, which increases toward the end of the sleep cycle, simply serves as a convenient transition between sleep and waking.

McNamara disagrees. "There is such an enormous amount of metabolic energy put into REM sleep that it would be really surprising if had no particular function," he says.

Recent studies have shed light on some of the benefits of sleep.

Last fall, researchers at Harvard Medical School and the University of California at Berkeley took 26 healthy young people and deprived half of them of sleep for 36 hours, then took images of their brains. They found that the sleep-deprived group showed 60 percent more activity in the amygdala, the part of the brain that alerts the body to protect itself in the presence of danger, while the prefrontal cortex, which controls logical reasoning, began to shut down. That suggests that adequate sleep can help a person cope with emotional challenges, sorting out real threats from false ones.

In one of a handful of studies on sleep deprivation and animal sexual behavior, Scott McRobert, a professor of biology at St. Joseph's University in Philadelphia, has found that male fruit flies who do not get enough ZZZZs lose their ability to perform the elaborate songs and dances that normal males use to attract females. "They don't perform courtship behaviors as long as the normal male does, they don't do it as vigorously. It eliminates their ability to mate," said McRobert.

Charles Nunn, a biologist at the University of California at Berkeley, is using the Boston University database to examine whether animals that get more sleep can more easily defend against illness. "There are many different immune system cells involved in the regulation of sleep, and the immune system is very costly to maintain," says Nunn. "The idea is to see if there is a link between these factors on an evolutionary time scale."

McNamara, meanwhile, still holds out hope that the marriage of evolutionary biology and sleep research will bring the answer that has eluded scientists for the past half-century.

"REM sleep was discovered the same year the DNA molecule was decoded and revolutionized science and medicine," he said. "The payoff from the discovery of REM sleep has been nothing like that. . . . I believe the study of the evolution of sleep is going to reveal its function."