This thesis explores accounts of impaired consciousness in three groups of sources that spread out between the 5th/4th century BCE and the 2nd century CE, namely, a selection of treatises from the Hippocratic Corpus, some post-Hellenistic authors, mainly Celsus and Aretaeus, and a number of Galenic treatises. I looked for what we nowadays consider as the main prototypical clinical presentations of impaired consciousness (i.e. delirium, sleep and swoons) in the ancient texts, in order to analyse the way in which they are portrayed. On the one hand, I diachronically compared the different medical writers with each other in a thematic arrangement, and on the other, the authors were synchronically contrasted against their respective contemporary intellectual milieu. The thematic analysis showed that philosophical discussions concerning the mind, the soul and their interaction with the body strongly influenced these doctors’ ideas on consciousness. Particularly, debates on perceptions, which were topical in the post-Hellenistic authors, conditioned their conceptualizations of human functioning. The look at the specific authors emphasised the use of partial synonymy in the Hippocratic corpus, revealed some Epicurean/Asclepiadean influences in Celsus’ understanding, helped to characterise the eclectic method used by Aretaeus, and illustrated the scope of Galen´s comprehensive system. In summary, the study suggests that impaired consciousness was being actively debated and distinguished from madness, that there was a primeval idea of consciousness underpinning most authors’ understanding of the clinical presentations, and that such an idea often challenged the clear boundaries between health and disease, wakefulness and sleep, and even between life and death.