A network within the brainstem that plays a role in regulating the alertness, or arousal, of the brain and thus plays an important role in managing sleep-wake transitions. The pathways included in this system start in the upper part of the brainstem and extend through the thalamus, and then on to the surface of the brain, called the cerebral cortex. The areas deep within the brain that function as part of this system include: midbrain reticular formation, mesencephalic nucleus, thalamic intralaminar nucleus, dorsal hypothalamus, and tegmentum. If this system is damaged bilaterally, it may result in deep sleep, lethargy, or coma. Dysfunction may also have a role in narcolepsy, schizophrenia, and other conditions.