Like their forerunners, post-Hellenistic doctors also grappled with the unclear boundaries between healthy versus pathologic sleep, and consciousness-unconsciousness. Furthermore, they incorporated new diseases and redefined others - like lethargy - that were specifically associated with this process. Celsus considered sleep as all-or-nothing phenomenon, without recognising different depths. Regarding mental capacities, he subsumed most of them in his idea of mens/animus. Aretaeus, on the other hand did conceive different depths of sleep, and his eclectic method enabled him to find alternative pathophysiological explanations to characterise several of its main features. Similarly, although his organization of mental capacities varied according to what he was explaining, the opposition gnômê-aisthêsis was important in his idea of mind.

Some Hippocratic doctors regarded sleep as a healthy process, and some as a pathological one; some of them struggled to distinguish between hallucinations and nightmares, and some between deep dreamless sleep and total loss of consciousness. This chapter explores how different treatises from the Hippocratic corpus navigated these ambiguities, how they explained different depth of sleep (i.e. different levels of consciousness), and how such understanding relates to their views on mental capacities (which they subsumed in concepts such as phronesis, sunesis, gnômê, and nous).

In face of the difficulty of establishing clear biological boundaries between sleep and the other forms of impaired consciousness, the sociological and anthropological analyses can provide hints as to where those limits were set in real life. The terminological analysis suggested a common feature that persisted throughout the different authors and periods: different levels of consciousness (from drowsy to hyperactive, and from delirium to koma) where always related to the impairment of mental capacities, regardless of the way in which each medical writer grouped or understood them.

Galen conceived sleep and wakefulness as a continuum that depended on the mixture of qualities within the ruling part of the puschê (the hêgemonikon) located in the brain. Naturally, in his system whenever pathological sleep occurred the doctor needed to determine if the brain was affected directly or by sympathy (from another organ), and the precise imbalance of qualities that needed to be counteracted by their opposites. His idea of mind was very accurately and hierarchically structured: it resided in the logical part of the soul, located in the brain, and several diseases with impaired consciousness compromised its normal functioning.