Although it's true that at any given moment all of the brain's regions are not concurrently firing, brain researchers using imaging technology have shown that, like the body's muscles, most are continually active over a 24-hour period. "Evidence would show over a day you use 100 percent of the brain," says John Henley, a neurologist at the Mayo Clinic in Rochester, Minn.
Even in sleep, areas such as the frontal cortex,
which controls things like higher level thinking and self-awareness, or the somatosensory areas, which help people sense their surroundings,
are active, Henley explains.
Source:
http://www.scientificamerican.com/ar...rcent-of-brain