Medically reviewed by
Dacelin St Martin, MD
Triple board-certified in Sleep Medicine,
Internal Medicine, and Pediatrics.
Our brains are exposed to many different sounds and scents all day long. How do our brains respond to these various stimuli? And can loud noises or strong smells wake you up from sleep?
During wakefulness, your brain produces alpha, beta, and gamma waves. Conversely, theta and delta brain waves dominate during sleep.
When you hear a sound while you’re sleeping, the power of delta and theta waves increases so your brain can respond to noise during sleep. However, the decrease in alpha-beta power required for your brain to understand and memorize the sound doesn’t happen while you’re asleep.
That’s why although sounds can wake you up from sleep, you cannot remember things you’ve heard during your sleep.
On the other hand, scents stimulate mainly beta and gamma brainwaves that predominate while awake but not while sleeping. Consequently, the brain cannot respond to odor while sleeping, and smells cannot provoke you to wake up from sleep.
The human brain is constantly active, even when we’re asleep. However, the activity of different brain waves varies between wakefulness and sleep states.
Four typical brain waves are produced in different parts of the brain: alpha, beta, theta, and delta.
When awake, the brain’s electrical activity is characterized by small amplitude high-frequency (>20 Hz) brainwaves. As you fall asleep, this activity transitions into higher amplitude and lower frequency brain waves until it reaches frequency <10 Hz in a deep sleep.
Alpha waves have a frequency between 8-13 Hz. These waves are most noticeable when the person is awake but resting with closed eyes. Alpha brain waves disappear when you’re asleep or while concentrating on a specific task.
Beta waves are high frequency (13-40 Hz) low amplitude brain waves usually observed during awake states. Beta brain waves typically take over when performing tasks requiring concentration, conscious thought, and logical thinking.
Theta waves have a frequency between 4-7.9 Hz and appear intermittently during sleep. Whereas the frequency of delta brainwaves ranges between 0.1-3.9 Hz and typically occurs during deep sleep.
Another type of brainwave is the gamma wave, the fastest type of brainwave (30-80 Hz), and they usually occur when you’re conscious and highly alert.
Briefly, when you’re awake but resting, alpha waves take over your brain. When your brain is engaged in activities that require attention and processing, beta waves replace the alpha waves. During deep sleep, delta waves predominate the brain.
The Brain’s Response to Sound
Sound first reaches the cochlea in your ear and travels through your brain to your auditory cortex, where the sound signal is recognized and memorized.
When your brain receives sound information, specific processes can let your brain know which information needs to be focused on and which can be ignored.
For example, the same happens when conversing in a crowded place. Your brain receives sounds from every direction, but you can concentrate on the things being said to you while your brain shuts out the background noise.
When your brain expects a distraction, like background noise, it reduces the power of alpha and beta waves, helping the brain prioritize important sounds that need to be understood and memorized.
The Brain’s Response to Sound During Sleep
Different sound stimuli can increase the power of theta and delta brain waves which are predominant during sleep. Thus, your brain can respond to and process different sounds while asleep.
A recent study published in Nature Neuroscience showed that the brain’s response to sound during sleep was similar to its response during wakefulness.
The study reports that sound can travel just as effectively from the ear to different regions of the auditory cortex, even during sleep.
The main difference was that the alpha-beta wave power after sound reached the auditory cortex did not decrease during sleep. Therefore, your brain receives sound when you’re asleep just as well as when you’re awake.
Nonetheless, when the alpha-beta power does not decrease, your brain will not be able to process and understand the sounds you’re hearing.
This neurological response can explain why although you can still hear sounds while you’re asleep, your brain isn’t able to further process or memorize the sounds while you’re sleeping.
The Brain’s Response to Smell
While noises like a bang or thunder can jolt you up from sleep, the smell of a fresh pot of coffee or something burning cannot awaken you from a deep sleep.
A study examined the effects of both a pleasant and an unpleasant smell during sleep. The results revealed that odors or scents could not arouse you from sleep.
But what’s the difference between sound stimuli and odor stimuli?
Researchers showed that the brain responds to different scents through theta, beta, and gamma waves.
Beta and gamma waves are high-frequency waves that mainly occur while awake. Moreover, theta waves only occur intermittently during sleep.
When awake, smells enter your nose, pass through your olfactory nerve, and reach your cerebral cortex. Beta and gamma brain waves become involved, and your brain can identify and memorize the scent.
During sleep, delta waves are prevalent in the brain. While sound stimulation increases the power of delta waves, odors and scents do not affect this type of brain wave.
Meanwhile, the absence of these brain waves during sleep can be why your brain cannot identify smell or scent while you’re sleeping.
Since sound can affect the brain during sleep, it’s no surprise that sleep disturbances are associated with noise pollution.
Poor sleep can cause short and long-term health consequences both physically and mentally. Therefore, sleep disruption caused by environmental noise can negatively affect your health.
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