Why do we zone out?

Have you ever been in class and your eyes zone, does your mind go completly blank? Well when your spacing out like that whats going through your mind is a lot of things. But most importantly our brain stops paying attention and goes blank. That doesnt mean your brain isnt working, it means that your brain is spacing. It could happen for a vary of reasons, lack of sleep, not eating enough, or your just out of it. If you zone out often that could be because your not exercising your brain enough so it does not have the power to keep thinking and processing. When your brain begins to feelk a lack of sleep you begin to shut down and stare off, no one knows how the brain does that it just stops thinking.