Sleep Filter

When you are asleep, reticular formation (a set of interconnected neurons in the brain) modulates the passage of sensory input through the thalamus to the cerebral cortex. It decides what stimuli are important enough to be processed. The humming of electricity, the feeling of your underwear against your butt, and the smell of the wet dog in your house are ignored. It has determined that these things are not dangerous and you can ignore them. A siren passing in the distance, a bomb going off, or a bright flashing light will attract your attention because it is different and considered important. Some stimuli might wake you up but if it isn’t important enough you will fall back asleep without even remembering (a light turning on in the hallway or a dog barking). Others might keep you up for hours (a door slam, gunshot, or the smell of smoke).

It all depends on the type of stimulation and how important your brain decides it is for you to be awake.