Title really. If it just me that experiences this then I’ll be amazed
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The simplest answer is that your brain filters it out. There are many sounds you don’t “hear” throughout the day until it’s pointed out. It’s a way of protecting you from sensory overload. It’s also how you don’t see your nose all day even though it’s constantly in your peripheral vision. Your brain filters it out.
If you’re autistic like me though the brain doesn’t filter it out and you’re constantly aware of everything all the time which leads to sensory overload and burnout.
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Your brain has an incredible capability to filter out rhythmic and expected stimuli. Neurons generally function in 2 modes- make things more active (excitation) or make things less active (inhibition). When you fall asleep, your body is inhibiting a whole bunch of things, particularly your motor system- its because of this that sleep paralysis occurs, part of your cortex “wakes up” so that wakefulness & consciousness are occurring is but the inhibition on your motor system is still there so you can’t move.
To make this more “sound oriented,” an example of the brain filtering out expected noise – your own foot steps. People who get hearing aids and cochlear implants often complain of how loud their footsteps are! Its something we don’t consider because our brains have a LOT of experience filtering out those sounds, but its a very precisely tuned system. I imagine that these two system interact when you sleep- inhibition of auditory perception (neurons that fire when a sound happens and you can “perceive” it) and inhibition of neurons that are responding to the particular input that is “fan.”
FYI- falling asleep isn’t gradual. Its a distinct, marked moment.
The brain may be good at a lot of things, but it’s great at one! “Pattern recognition” and because of that it can “filter out” patterns (visual or audio), as required.
i.e. your eyes each have a “blind spot”, the area at the back of each eye where all the optical nerves go to the brain. you’re brain can “edit out” those blind spots from your vision so you don’t notice them.
With audio it’s sort of the same. Your brain can choose what to hear and not. It tends to prioritise “new and unexpected” sounds over “constant white noises” (leaves rustling in the wind = safe. twig snapping = possible danger!), so repeating sounds, like your fan, can be easily filtered out, but you’ll still hear your alarm going off to wake you up.
Your brainwave state has changed from alpha, relaxed and awake, to theta, entering first stages of sleep. As it moves into deeper theta, your brain functions differently as you lose waking consciousness, enter REM sleep (dreaming), and eventually moves into delta, deep, dreamless sleep.
Here’s more about the fuzzy boundaries between hearing and not-hearing, during the liminal states between waking and sleeping:
[https://theconversation.com/brains-can-make-decisions-while-we-sleep-here-they-are-in-action-31716](https://theconversation.com/brains-can-make-decisions-while-we-sleep-here-they-are-in-action-31716)
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Ears convert soundwaves to a signal that your brain can read, and that is how you hear. When you sleep your brain turns off. This means two things:
1. Your brain is no longer actively handling the signal from your ears. You can think of your brain as an amplifier for the signal.
2. Your memory is also turned off. Everything that your brain handles involving sensory input is just not committed to memory, so you plainly can’t remember the fact that you heard the fan all night.
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