The reticular activating system (RAS) is a component of the reticular formation in vertebrate brains located throughout the brainstem. Between the brainstem and the cortex, multiple neuronal circuits ultimately contribute to the RAS. These circuits function to allow the brain to modulate between slow sleep rhythms and fast sleep rhythms, as seen on EEG. By doing this, the nuclei that form the RAS play a significant role in coordinating both the sleep-wake cycle and wakefulness. The groupings of neurons that together make up the RAS are ultimately responsible for attention, arousal, modulation of muscle tone, and the ability to focus.