Have you ever noticed how some people have a hard time waking up?

This is because they are in deep sleep, which usually requires us to be really persistent if we want to wake them up.

The reason for this is because when you go to sleep, the thalamus, a part of your brain, also goes to sleep. Thalamus is a sensory middleman, receiving information from your sensory organs and then sending it to your cortex for processing. When thalamus is off (i.e. during deep sleep) some sensations never get passed on to the cortex. 

The thalamus is responsible for recognizing and labeling sights and sounds. Yes, that's right – not smells. This is why people who are passed out can be woken up by the smell of salt.