Reticular activating system:

The general region of the reticular activating system is highlighted in red.

sometimes also known as the ascending arousal system, the reticular activating system (RAS) consists of nuclei and tracts that are involved with regulating wakefulness, arousal, and some aspects of sleep. The primary components of the RAS are found in--or arise from--the reticular formation.

Learn more:

Know Your Brain: Reticular Formation