reticular formation

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A diffuse network of white longitudinal nerve fibers interspersed with gray matter, located in the brain stem, that regulates various autonomic functions, such as sleep and waking.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Part of the brain that is involved in actions such as awaking/sleeping cycle, and filtering incoming stimuli to discriminate irrelevant background stimuli

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a complex neural network in the central core of the brainstem; monitors the state of the body and functions in such processes as arousal and sleep and attention and muscle tone