Brain Studies May Reveal the Purpose of Sleep

By Tina Hesman Saey, Science News

In a lab at MIT, a small black mouse named Buddy sleeps alone inside a box. A cone resembling a satellite dish sits atop his head. But the dish doesn’t receive signals from outer space. Instead it sends transmissions from deep inside Buddy’s brain to a bank of computers across the room.

Scientists like Jennie Young eavesdrop on the transmissions, essentially reading Buddy’s mind, or at least that part of his mind occupied with a recent trip along a Plexiglas track littered with chocolate sprinkles. Young and her colleagues in Susumu Tonegawa’s laboratory are monitoring nerve cells inside the hippocampus, one of the brain’s most important learning and memory centers. Some of the cells in the sea horse–shaped hippocampus fired bursts of electrical energy as Buddy moved along the track. As he sleeps in his black box, those same cells spark to life again, replaying progress along the track in fast-forward or rapid reverse.

By recording the slumbering Buddy’s brain cell activity, the scientists hope to glean clues to one of biology’s greatest mysteries: the reason for sleep. Although sleep is among the most basic of behaviors, its function has proved elusive. Scientists say sleep’s job is to save energy, or to build up substances needed during waking or to tear down unneeded connections between brain cells. Some emphasize sleep’s special role in learning and memory. Others suggest that sleep regulates emotions. Or strengthens the immune system. And some scientists believe sleep is simply something that emerges naturally from having networks of neurons wired together.

“There are as many theories of sleep’s functions as there are sleep researchers,” says Mehdi Tafti, a geneticist at the University of Lausanne in Switzerland.

None of the many models for why people (and other animals) sleep can explain all of its complexity, says Robert Stickgold of Harvard Medical School in Boston. He equates proponents of the different sleep theories to blind men describing an elephant. It’s a snake, or a tree or a wall, depending on which part of the elephant the men touch. Similarly, the answer to sleep’s function seems to depend on what approach a given researcher takes. And each proposed idea contains inconsistencies that keep other sleep researchers from embracing it.

“There’s no one theory that has enough unified evidence for it to be widely accepted,” says Paul Shaw of Washington University in St. Louis.

Many sleep theories have been widely tested, though. Using brain wave recordings, genetic analyses, word tests, video games and various other methods, researchers have uncovered many of the pieces to the puzzle of sleep, even if they don’t yet all fit together.

Asleep and fired up

Not knowing why humans spend a third of their lives unconscious hasn’t prevented scientists from describing five different stages of sleep from recordings of brain waves. Stage one, marking the transition between awake and asleep, is shallow. Stage two, which lasts the longest, features two forms of brain waves known as spindles and K-complexes (SN Online: 5/21/09). Stages three and four are the deepest, often referred to collectively as slow-wave sleep. Fifth is REM, the stage accompanied by rapid, jerky eye movements.

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