Consciousness consists of two components: arousal and awareness of the environment and self. Arousal refers to the behavioral continuum that occurs between sleep and wakefulness, while awareness to the content of consciousness (Laureys, 2014). In normal physiological states, with the exception of REM sleep, the level of arousal is positively correlated with awareness (Laureys, 2014) such that arousal is graded and measured by the degree to which it is diminished. This conceptualization appears to have led experts, including the DSM-5, to consider delirium as a condition to not associated with impairments in arousal (Schiff and Plum, 2000, American Psychiatric Association, 2013). However, arousal can be both diminished and heightened, not unlike the performance of an old-fashioned incandescent bulb at different voltages, giving barely sufficient light at low voltages and brightening up unbearably at high voltages (MacIsaac et al., 1999). Delirium is associated with both hypo-arousal and hyper-arousal (Han et al., 2017). A small but significant proportion of delirious older patients (10%; n = 155) in the emergency department in this study had normal arousal at the time of the short arousal assessment (Han et al., 2014).