This chapter presents sleep within a system of opposing tensions (consciousness-unconsciousness, health-disease), and in the midst of extra biological debates, particularly anthropological and sociological. Such tensions and debates illuminate how understanding sleep can be useful to apprehend ancient doctors’ ideas about the mental capacities that are compromised in impaired consciousness.

Contrary to mainstream scholarship’s opinion, the Hippocratic corpus presents many cases of impaired consciousness, but only a few of mental illness. By looking at three study cases, this chapter describes how these doctors understood conditions where patients act weirdly or were not their usual selves, and how they construed the notion of disease.

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.

The analysis of total loss of consciousness illustrates the varied ways in which the different authors resolved - in their corresponding periods and contexts - the tension between body and soul. Despite their diverse approaches, all the medical writers under scrutiny took for granted the existence of a soul, its intervention in this kind of conditions, and its bonds to the body as determiner of the clinical presentation. Particularly, they grappled to organise the mental capacities and explain how they were affected in the different forms of impaired consciousness.

Unlike mental disease, which presupposes a strongly theory-laden concept, impaired consciousness or delirium is currently conceived in medicine as a cluster of symptoms. This chapter contrasts these two constructs, and discusses our current idea about the notion of disease.

Total loss of consciousness is nowadays mostly framed as a global alteration of brain activity. In antiquity, doctors often alluded to this symptom with compound terms of psuchê or anima, and they understood the body and the soul to be involved - to different extents - in the phenomenon. Consequently, by exploring how they conceived this condition, it is possible not only to better understand their idea of consciousness, but also to get a hint of how the envisaged the relation between body and soul.

Unlike the other presentations, Galen associates both forms of total loss of consciousness, lepipsuchiê and sunkopê, with an affection (whether direct by sympathy) of the heart. Namely he localises the problem in the seat of the spirited part of his tripartite soul. Nevertheless, the loss of pneuma provides a link with the affection of the rational soul, whereby both conditions cause total loss of movement and perceptions. The idea of the soul as a life principle is also present in this author, because although total loss of consciousness is associated with the temporary depletion of pneuma (and finishes when it is replenished), the separation of the soul does cause death.

Post-Hellenistic authors took some Hippocratic ideas and terminology to build their own theories about the different forms of losing consciousness, and about the relationship between body and soul. Also, they presented a clearer distinction between the two forms of total loss of consciousness (to the extent that they described a new disease, where the body was primarily affected but not the soul). Celsus’ description of fainting suggests that his idea of soul was influenced by Epicurean corpuscular theories with a rational and an irrational component. Aretaeus, in his turn, was majorly concerned with the mechanisms that produced fainting, where he included a tangle of ideas that included loss of heat, loss of tension, affection in the blood or in the heart, and sometimes, the separation of the soul. However, his idea of psuchê was rather erratic, and his way of organizing mental capacities was not consistent throughout the treatise.

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.