The lower portion of the brain contains an area known as the Reticular Activating System. It keeps the higher brain centers alert and ready for input. There is some evidence that this area is not working properly in ADHD, and that the brain is, in effect, " going to sleep". Hyperactivity is really the brain's attempt to generate new stimulation to maintain alertness. For more information,see : Kinomura et. al., Science, Jan. 26,1996, Vol. 271, pp. 512-515