Sometimes the best science is done by looking at the obvious — the daily behaviors that we do — to solve a mystery. And watching almost any sleep-deprived parent and his or her young child will give you an instant insight into two powerful mechanisms that underlie human sleep, one of neuroscience’s greatest mysteries, as the parent rocks his or her infant back and forth, crooning a lullaby. These are two behaviors that have probably been used by humans seeking sleep for their children (and themselves) since humans first appeared, and both are driven by our ears.
It may seem odd to think that rocking someone to sleep has anything to do with your ears, but your ears contain two sensory systems — the auditory system for hearing sounds, and the vestibular system, which normally underlies balance. Both are driven by similar types of sensors, called hair cells. These are tiny, tufted cylinders with tips that wave back and forth in fluid -filled chambers, each responding to different types of motion: oscillations of pressure waves that are translated into sound, and slower, linear or angular motions based on how the head moves (which drives balance).
Normally, the two systems are separate, projecting to different areas of the brain and helping define different ways in which we sense our environment. But both systems can overlap under certain circumstances. Managing sleep is one of the most profound ways in which they interact.
Balance and the Vestibular System: your brain’s way of processing movement
One of the things that the balance system does is let us know when things are wrong with the way we are moving. Standing on a boat in high seas, your vestibular system will tell you that the world is moving up and down at one rhythm, while your stomach and eyes are experiencing movement in different directions. Radical motions that separate what your inner ears and your eyes tell you are happening trigger nauseogenic motion sickness. But slow down the motion, make it almost regular, slower, and gentler, and your inner ears do something odd. They put you to sleep. Whether it’s a baby rocking gently or a passenger in a car, bus, or train, gentle vibrations transmitted through your body to your inner ears trigger another form of motion sickness. It’s called Sopite syndrome, and, rather than making you want to lose your lunch or die, it activates your global sleep network. But it’s not always convenient to drive your child around on bumpy roads to get her to drift off, or possible to rock your baby in a quiet environment.
Low frequency sounds: feel the beat
This is where the other part of your ears can help. While normally there is no cross talk between your hearing and balance system, high pressure/low frequency sounds can trigger responses in the balance-sensitive hair cells in your ears. This is why most effective dance music pumps up the bass, hijacking your sense of hearing to trigger motor responses. In other words, rhythmic deep bass sounds make you feel the beat and want to match it with body motion.
But the truth is, we don’t hear very low frequency sounds very well, and even sound pressure levels of 70 dB — what would seem like a moderately noisy street or bar scene at higher frequencies — are perceived as relatively quiet at lower frequency. And here lies the opportunity. By providing semi-regular, low frequency sounds that are audible, but not loud enough to make you want to dance or run away, you can trigger Sopite syndrome and provide a gateway to sleep. And by providing soft, regular sounds in a familiar register (like the universal aspects of lullabies, which stretch back more than a thousand years), you block out the distracting environmental sounds that can interrupt falling asleep.
The Listening Program® SLEEP uses those combinations of sounds so that your ears tell your brain it’s time to sleep. You may not feel like you’re being rocked like a baby, but your brain will get that impression. And it’s all due to the two functions of your ears.
So the next time you do bundle your cranky child into the back seat of your car and finally get her to fall asleep, remember to thank her ears. Just make sure you don’t let yourself get lulled by those very same inputs to your own brain.