What is the reason for deep sleep when some people sleep despite all the noise?

Because they sleep more deeply and have more brain activity known as sleep spindles.

Everyone's sleep is different, although we all go through the same four stages of non-REM sleep, and sleep several periods of REM each night.

In the awake brain, a large area called the thalamus acts as a station for sounds, sights, and other stimuli that come in, but during sleep it helps to suppress them.

Patterns called sleep spindles, which can be seen using electroencephalography, reveal the onset of non-REM sleep.

Recent research has found that truly deep sleepers - those who will "sleep through anything" - have more sleep spindles than others.