Slow-wave sleep

The science of sleep was one of my earliest column topics, way back in 1991.

And why not? After all, as I pointed out then, sleep is so important birds, fish, reptiles and mammals all do it, we spend a third of our lives doing it, and if we don’t do it, we die. Like eating and breathing, it is, literally, vital.

But as I also noted in 1991, we don’t exactly know why it’s so vital.

A scientist in Wisconsin, though, thinks he has a handle on the question—and he’s developed a laboratory tool that might not only help him test his hypothesis, but also help treat people with sleep disorders.

There are several distinct stages of sleep, each marked by its own specific type of electrical activity in the brain. While we’re nodding off, the brain produces regular alpha waves, seven to 12 times per second. Next comes a stage of light sleep, during which we produce theta waves a three to seven cycles per second, interrupted by bursts of activity called sleep spindles, at 12 to 15 cycles per second.

Next comes a period of regular, high-amplitude delta waves at only one to two cycles per second, with a few sleep spindles. Finally sleep deepens to the point that the brain produces wave patterns similar to those occurring in a coma, at one to three cycles per second.

Occasionally the brain suddenly erupts with activity, similar to the waking state. During REM (rapid-eye-movement) sleep, our eyes move rapidly under our closed eyelids, our heart rate, breathing rate and blood pressure, which dropped with our body temperature as we fell asleep, all go up, and we dream. (Fortunately, our bodies remain effectively paralyzed.)

All these stages occur in a regular 90-minute cycle, usually four to six times a night. But which one explains sleep’s power to restore our mood and renew our ability to learn, think and remember?

Recently, research has been pointing to the slow wave activity of the deepest stage of sleep. We know, for example, that sleep-deprived people have larger and more numerous slow waves once they fall asleep, and that those waves weaken over time, presumably because the need for sleep has eased.

Now Guilio Tononi, a professor of psychiatry at the University of Wisconsin-Madison School of Medicine and Public Health, has discovered that it’s possible to stimulate slow wave activity in sleeping volunteers whenever he wants via transcranial magnetic stimulation, which sends a harmless magnetic signal through the scalp and skull.

This could someday lead true “power nap” where you could get the benefit of deep sleep in a shorter period of time. It could be used to treat insomnia and other sleep disorders, and possibly even chronic fatigue syndrome, which a recent study at the University of Michigan discovered is associated with blunted slow wave activity when the sufferer is sleep-deprived.

Tononi acknowledges those possibilities, but he’s primarily interested in generating slow wave activity on demand so he can test his hypothesis about why we need sleep.

We know we consolidate memories during sleep. Many neoruscientists think we do that by somehow rehearsing the day’s events in our sleep, but Tononi thinks the mechanism is different. He believes sleep allows the connections between nerve cells in the brain, called synapses, to relax.

When we’re awake, we “observe and learn much more than you think,” he says. “Tons of things are leaving traces, changing the synapses, mainly by making them stronger.” But there’s a cost to that: “Synapses require proteins, fats, space and energy.”

At the end of the day our brains are full of traces of memories, Tononi thinks. During slow wave activity, all of these synaptic connections are downscaled, freeing up energy, space and nutrients for new memories during the next waking period.

Slow waves, in his words, “clear out the noise to make sure your brain does not become too much of an energy hog, a space hog. By morning, you have a brain that is energy-efficient, space efficient, and ready to learn again.”

“Let me sleep on that!”, it turns out, is more than just a delaying tactic: it’s also scientifically sound.

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