Constable: Why noises that lull some to sleep wake others

I'm fast asleep, blissfully snoring away, when an odd sound jars me from my slumber at 4:16 a.m.

It's coming from my wife's new HoMedics Sleep Solutions Portable SoundSpa Mini. The device's "white noise" - a constant heterogeneous mixture of sound waves extending over a wide frequency range - keeps me from falling back to sleep. Cheryl bought that battery-operated noisemaker in large part to counteract my occasional snoring, which consists of an intermittent mixture of sound waves extending over an annoying frequency range at varying volumes and rhythms.

The night she brings home her $12.99 find, my wife uses the "ocean" setting, which reproduces the sound of waves lapping at a beach in the same way that I reproduce the sound of David Bowie when I sing "Space Oddity" in the shower. It doesn't wake me, but it doesn't fool Cheryl into thinking we're sleeping in a seaside villa. The "gentle rain" setting makes us both worry that our basement is flooding. The "Everglades" setting reinforces our fear of closing our eyes in a place loaded with alligators. Just as all light frequencies combine to form white light, all sound waves combine to form white noise.

With our eyes closed and our bodies at rest, we generally don't wake up because of something we touch, taste, see or smell. Hearing is different.

"Your sense of hearing works even while you're asleep. It's our evolutionary alarm clock, letting you startle awake and pay attention to things you can't see in the dark to keep you from being eaten by them," emails Seth S. Horowitz, a neuroscientist and author of "The Universal Sense: How Hearing Shapes the Mind."

"Your ears are very sensitive to sudden sounds, transients," Horowitz explains. "Sleep machines create broadband noise to try and mask environmental sounds … hopefully blocking these transients and letting you keep sleeping."

My wife and I have learned to sleep through the blaring sirens of ambulances rushing to a hospital six blocks from our house. But my mom, who has spent 89 years sleeping in rural Indiana, where you never hear an ambulance, finds sirens jarring. She can, however, sleep through the racket of a rattling 18-wheeler rumbling past her farmhouse at 70 miles per hour, or even the jabbering on late-night MSNBC reruns of "All In with Chris Hayes." Why does white noise meant to lull people to sleep bug me? And why does the sound of my snoring irritate my wife, our kids and people sleeping in nearby houses?

"Your brain is REALLY good at 'normalizing' environments over time," Horowitz says. "This is due to some really basic features of how neurons and brains operate, including 'habituation.' Habituation occurs when a stimulus is presented multiple times. Eventually you will actually stop perceiving it consciously (and even individual neurons will stop responding to it). But if you suddenly make a minor change to it (e.g., change the pitch, loudness, rhythm, even change the time it occurs) your brain will re-categorize it as different and suddenly you will snap to attention."

According to a survey by the National Sleep Foundation, 72 percent of Americans think a quiet room is important to sleep. But 5 percent use a sound machine, 11 percent sleep with a TV on and many others use a fan or air purifier or fall asleep to music. Some use earplugs or take drugs. Another 10 percent sleep with a pet, which probably makes some noise. Parents often resort to using some noise-emitting device to calm a baby. The National Geographic documentary "Sleepless in America" says that 40 percent of Americans are sleep-deprived.

YouTube offers all-night sleep videos featuring rain, waterfalls, space noises, washing machines, trains, planes, buses, hair dryers, vacuum cleaners, electric fans, dishwashers, hair trimmers, a sandstorm, car tires on a gravel road, a Harley-Davidson motorcycle idling, kittens purring, a ship's engine room, a grandfather clock, a running toilet, sheep, pigs, cows, horses, crickets, turkeys, flamingoes and "12 Hours of a Bull."

If you can sleep through bull noises, a little snoring isn't going to bother you.