system
Given the following post, respond as if you're commenting on Reddit:
human
How does your brain determine whether a sound is loud enough to wake you up?

It must be different for people, since there are light and heavy sleepers.
gpt
Sound feeds into the brain's arousal pathways, in the reticular activating system. It is a weak input (vision is much stronger), but it is there. The sound inputs will activate reticular activating system neurons, which will "turn on" the brain by sending acetylcholine to the thalamus and activating the Nucleus Basalis to send acetylcholine to the cerebral cortex. The thalamus is likely to be the more important target, though.  

The reticular activating system is in different states of suppression in different phases of sleep. The sound level necessary to wake someone changes dramatically depending on where they are in their sleep cycle.