How does the human brain distinguish between noise and alarm when asleep?

Throughout the ages, even the simple act of striking the time has posed great challenges to humanity, and we have tried to solve this problem through well-designed inventions. The ancient Greeks and Egyptians invented sundials and towering obelisks to mark time with shadows moving with the sun. Dating back to around 1500 BC in the United States, hourglasses, water clocks, and oil lamps were invented to calibrate the passage of time with the movement of sand, water, and oil.

From these early inventions, there were some initial attempts to create a kind of morning alarm clock – such as a candle clock. These simple devices from ancient China have nails embedded in them, and when the wax melts, the nails are released, and for a certain amount of time, the nails make a loud noise in the metal tray, waking up the sleeper. (Why can't we remember our dreams?)

But these shoddy inventions are unavoidable and unreliable. As a result, until more precise mechanical inventions were created, humanity had to rely on another, more innate way of keeping time: our own biological clock.

Melinda Jackson, a senior research fellow in sleep and psychology at the Royal Melbourne Institute of Technology University in Australia, said humans have two physiological processes that make up our natural sleep and wakefulness patterns: homeostasis and circadian rhythms. "The main principle of homeostasis is a signaling process controlled by the hypothalamic region of the brain," Jackson told the journal Living Science.

Then, "when we fall asleep, the drive to sleep dissipates throughout the night" — a sign it's time to get up, she says.

On top of this, the circadian rhythm – also controlled by the cells of the hypothalamus – is a parallel process that regulates the phases of sleep and alertness throughout the day. This process is also affected by light and darkness, which means that the times of alertness and sleepiness usually correspond to the morning light and the darkness of the night, respectively. In the era before the advent of alarm clocks, Jackson said, people probably woke up like this, waking up to the accumulation of sleep time and the rising sun.

Because the sound is louder, the alarm clock is more pleasant.

I can't tell the difference, and I'll only be noisy when I'm asleep.

I can't tell the difference, as long as I am noisy, I will wake up.

In fact, it is indistinguishable, and when the noise is loud, it is awake.

After falling asleep, I don't hear any noise.

It may be loud and noisy to wake you up on your own.

It is impossible to discern, as long as the sound is loud enough, the person will wake up.

It's basically unrecognizable, and if you can recognize it, you're sleeping lightly.

I can't tell the difference, when I'm asleep, any sound is noise.

As long as the sound is loud, no matter what the sound is, it will wake up.