Despite the many definitions through which consciousness can be described, from an anatomical point of view it can be seen as the integration of two main components: the state of Alertness and the Awareness content. Studies concerning consciousness can be better carried out analyzing the Alertness component, as its state results more detectable compared to awareness. Consciousness is characterized by the activation of the brain stem and the cerebral cortex due to the thalamus. Its main functions rely on the definition of the sleep-wake cycle and, at the same time, on different sensory and cognitive applications. The study of wakefulness allows to distinguish intermediate states between sleep and wake, and they might not always be physiological.