Insomnia and fatigue are two sides of the same coin: if a person is unable to sleep at night, they are usually fatigued during the day. However, some people get eight to 12 hours of sleep a night and still have fatigue.
The top part of the brain stem is called the mesencephalon, and it determines our sleeping/waking patterns. The specific term is the "mesencephalic reticular activating system" which is a three-dollar phrase for