After listening to Stephen's antisemitic song in Ithaca, Bloom ponders a multitude of conditions capable of producing dangerously aberrant behavior in human beings. The list concludes with "hypnotic suggestion and somnambulism." As subsequent paragraphs explain, he is thinking not of hypnosis in the usual sense but of several different sleep disorders, known collectively as parasomnias, which he and his daughter Milly have experienced. Bloom's hunch about how closely these conditions are related is spot on, but his fear of them (expressed later in the chapter by a reference to "malignant agencies") is somewhat exaggerated. 

In his popular book The Body: A Guide for Occupants (2019), Bill Bryson observes that human parasomnias include "sleepwalking, confusional arousal (when the victim appears to be awake but is profoundly muddled), nightmares, and night terrors. The last two are not easily distinguished except that night terrors are more intense and tend to leave the victim more shaken, though curiously victims of night terrors very often have no recollection of the experience the following morning. Most parasomnias are much more common in young children than in adults and tend to disappear around puberty, if not before" (272-73). Bryson omits to mention sleep paralysis, another important member of this class of disorders.

Ithaca makes clear that the pattern of sleep disturbances afflicting children applies well to Milly, who "at the ages of 6 and 8 years had uttered in sleep an exclamation of terror and had replied to the interrogations of two figures in night attire with a vacant mute expression." Milly seems to have suffered from some confusional arousal after she was aroused from her night terrors. The chapter does not say when Bloom experienced his own sleep disorders, but it identifies them very precisely: "From hypnotic suggestion: once, waking, he had not recognised his sleeping apartment: more than once, waking, he had been for an indefinite time incapable of moving or uttering sounds. From somnambulism: once, sleeping, his body had risen, crouched and crawled in the direction of a heatless fire and, having attained its destination, there, curled, unheated, in night attire had lain, sleeping."

It may seem odd to blame these strange, brief states on "hypnotic suggestion," but this is Ithaca. The Greek noun hypnos means simply "sleep," and Hypnos was sometimes seen as a god of sleep. Many purely sleep-producing chemicals (as opposed to ones that induce sleepiness as a side effect) are classed as "hypnotics," including melatonin, the hormone produced in the pineal gland in all diurnal mammals. Buried within the capacity to fall asleep, however, are some of the strange hybrid states of unconscious consciousness more commonly evoked by the word "hypnosis." Insomnia medications like Ambien (zolpidem), which is notorious for causing sleepers to do things of which they later have no memory (online binge shopping, driving vehicles, having sex) are also classed as hypnotics.

The first of these three conditions—waking up and for a few moments not knowing where one is—is probably another form of "confusional arousal." The second––being "for an indefinite time incapable of moving or uttering sounds"––is commonly known as sleep paralysis. It usually occurs while falling asleep or beginning to wake up. Victims are fully conscious and aware of their surroundings, but they cannot move or speak. The condition is thought usually to last no more than a couple of minutes, but it can seem longer because the bodily paralysis, sometimes accompanied by auditory hallucinations and feelings of pressure on the chest, can induce panic and even terror. For many centuries people attributed these experiences to incubi, succubi, witches, demons, jinns, and other malignant beings. Henry Fuseli's famous painting The Nightmare, which shows an incubus sitting on a woman's chest, is often supposed to represent the condition of sleep paralysis.

The third condition––"somnambulism," or sleepwalking––describes episodes when people get up and move around while still asleep. Joyce's prose reflects that state of near-unconsciousness: it is not Bloom himself, but "his body," that moved to the fireplace. The general public is more aware of sleepwalking than the other disorders discussed here, but all of them are quite common, all of them seem to be intimately related, and, as Bryson notes, most if not all of them occur more frequently in childhood than adult life. Bloom's scientific instincts are quite sound when he assumes that Milly's night terrors are a "cognate phenomenon" with his own sleepwalking, and it seems that he may have looked into advice on how to deal with this phenomenon it ever showed up in his daughter. In Circe he bends over the unconscious Stephen and says, "Eh! Ho! (There is no answer; he bends again.) Mr Dedalus! (There is no answer.) The name if you call. Somnambulist. (He bends again and hesitating, brings his mouth near the face of the prostrate form.) Stephen! (There is no answer. He calls again.) Stephen!"

These strange, and frequently alarming, features of the human ability to hover between consciousness and unconsciousness clearly have given Bloom some empathetic identification with his young daughter. They also have given him some apprehension about falling asleep. Later in Ithaca, after discussing Bloom's strategies for getting a good night's sleep, and noting that "he believed in the artificial placation of malignant agencies chiefly operative during somnolence," the narrator asks, "What did he fear?" The answer: "The committal of homicide or suicide during sleep by an aberration of the light of reason." The mortal fear experienced in sleep paralysis and night terrors has made Bloom wonder what kinds of dark forces may lurk just beneath the surface of normal consciousness.

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