Study puts us one step closer to understanding the function of sleep

Sleep remains one of the big mysteries in biology. All animals sleep, and people who are deprived of sleep suffer physically, emotionally and intellectually. But nobody knows how sleep restores the brain.

Now, Giulio Tononi, a professor of psychiatry at the University of Wisconsin-Madison School of Medicine and Public Health, has discovered how to stimulate brain waves that characterize the deepest stage of sleep. The discovery could open a new window into the role of sleep in keeping humans healthy, happy and able to learn.

The brain function in question, called slow wave activity, is critical to the restoration of mood and the ability to learn, think and remember, Tononi says.

During slow wave activity, which occupies about 80 percent of sleeping hours, waves of electrical activity wash across the brain, roughly once a second, 1,000 times a night. In a paper being published this week in the Early Edition of the scientific journal PNAS, Tononi and colleagues, including Marcello Massimini, also of the UW-Madison School of Medicine and Public Health, described the use of transcranial magnetic stimulation (TMS) to initiate slow waves in sleeping volunteers. The researchers recorded brain electrical activity with an electroencephalograph (EEG).

A TMS instrument sends a harmless magnetic signal through the scalp and skull and into the brain, where it activates electrical impulses. In response to each burst of magnetism, the subjects' brains immediately produced slow waves typical of deep sleep, Tononi says. "With a single pulse, we were able to induce a wave that looks identical to the waves the brain makes normally during sleep."

The researchers have learned to locate the TMS device above a specific part of the brain, where it causes slow waves that travel throughout the brain. "We don't know why, but this is a very good place to evoke big waves that clearly travel through every part of the brain," Tononi says.

Scientists' interest in slow waves stems from a growing appreciation of their role in sleep, Tononi says. "We have reasons to think the slow waves are not just something that happens, but that they may be important" in sleep's restorative powers. For example, a sleep-deprived person has larger and more numerous slow waves once asleep. And as sleep proceeds, Tononi adds, the slow waves weaken, which may signal that the need for sleep is partially satisfied.

Creating slow waves on demand could someday lead to treatments for insomnia, where slow waves may be reduced. Theoretically, it could also lead to a magnetically stimulated "power nap," which might confer the benefit of eight hours sleep in just a few hours.

Before that happens, however, Tononi must go further and prove that artificial slow waves have restorative benefits to the brain. Such an experiment would ask whether sleep with TMS leads to greater brain restoration than an equal amount of sleep without TMS.