I am wondering why people can’t hear when they are asleep? – Joanna
This is a fantastic question, Joanna!
During sleep, our body can decide to ignore sounds, movements and smells happening around us which might otherwise wake us.
This decision-making mostly happens in our brain.Sudowoodo/ Shutterstcok
Although our ears continue to work as usual, our brain acts as a filter and decides whether we should respond to the sound and wake up or continue sleeping.
If we wake up, then we can form a memory of having heard the sound, but if we don’t wake up then it’s as though we didn’t hear anything.
This is an extraordinary tool as it protects our sleep so we don’t wake up to everything happening while we sleep.
But it also doesn’t completely shut us off from the outside world which would be terrible for our survival.
Our brain responds to loud sounds
Louder sounds are more likely to wake us up than quieter sounds.
For example, a loud bang from someone dropping something in the middle of the night is likely to startle and wake us.
But we’ll probably sleep through the sound of a mosquito quietly buzzing nearby.LeManna/ Shutterstock
The type of sound matters too
Sounds that are either unusual or important to us are also more likely to wake us.
Our brain interprets unusual sounds as a threat and alerts us to that danger. This allows us to decide if we need to protect ourselves or run away if necessary.
Just imagine how useful and protective this would have been for our ancestors who likely slept in the wild surrounded by dangerous predators, such as lions and tigers!
Luckily, we don’t have to worry about sabre-toothed cats anymore, but it’s still useful to be aware of loud bangs or strange noises while we sleep so we or parents can respond.
Our brain is also more likely to wake us to sounds it considers important like our name.Cookie Studio/ Shutterstock
We will more readily wake when our name is called compared to someone else’s name being called out.
What about depth of sleep?
When we sleep we go through cycles consisting of light and deep stages of sleep.
We have about five to six sleep cycles each night, depending on how long we sleep.
During light sleep you will be woken more easily than during deep sleep.
We have more deep sleep in the first half of the night and more light sleep in the second half of the night.
This means that the sound of a crowing rooster that instantly woke us at the break of dawn, may have have been ignored by our brains early in our sleep period.
Finally, people have very different sensitivity levels toward sounds.
Background chatter in your house while you are napping might not wake you if you’re not sensitive to noise.
However, someone who is very sensitive to noise might find it unbearable to keep sleeping in this noisy environment.
If we are more sensitive to sounds, then our brain is more likely to make the decision to wake us.
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Authors: Gorica Micic, Postdoctoral research fellow, Flinders University