Arousal is a state of vigilance regulated by subcortical parts of the nervous system, especially connections between the nuclei of the amygdala, the hypothalamus and the brain stem.
These unconscious responses prepare the body for action. In terms of sleep/wake regulation, the arousal systems are those that have highest activity during wake, for example the aminergic (noradrenaline, 5-HT, histamine) systems.
The arousal systems inhibit, and are themselves inhibited by the GABAergic system emanating from the ventrolateral preoptic nucleus (VLPO), in a so-called “flip flop” arrangement that is stabilised via orexinergic activity.