SOMNAMBULISM (from Lat. somnus, sleep, and ambulare, to walk), or sleep-walking, the condition under which people are known to walk along while asleep, apparently unconscious of external impressions, return to bed, and when they awake have no recollection of any of these occurrences. Sometimes the actions performed are of a complicated character and bear some relation to the daily life of the sleeper. Thus a cook has been known to rise out of bed, carry a pitcher to a well in the garden, fill it, go back to the house, fill various vessels carefully and without spilling a drop of water, then return to bed, and have no recollection of what had transpired. Again, somnambulists have been observed to write letters or reports, execute drawings, and play upon musical instruments. Frequently they have gone along dangerous paths, executing delicate movements with precision.
Four types of somnambulists may be noticed: (1) those who speak without acting, a common variety often observed in children and not usually considered somnambulistic; (2) those who act without speaking, also well known and the most common type; (3) those who both act and speak, more exceptional; and (4) those who both act and speak and who have not merely the sense of touch active but also the senses of sight and hearing. The fourth class is the most extreme type and merges into the physiological condition of mesmerism or hypnotism (q.v.), and it is necessary here only to notice it in connexion with the subject of sleep. Many observations indicate that, at all events in some cases, the somnambulist engaged, for example, in writing, has a mental picture of the page before him and of the words he has written. He does not see what he really writes. This has been proved by causing persons to write on a sheet of paper lying on the top of other sheets. After he had been allowed to write a few sentences, the sheet was carefully withdrawn and he continued his writing on the next sheet, beginning on the new sheet at the corresponding point where he left off on the first one. Moreover, the somnambulist, by force of habit, stroked t’s and dotted i’s at the exact places where the t’s and i’s would have been had he written continuously on one sheet, showing that what he was conscious of was not what was before him, but the mental picture of what he had done.
The following table, modified from two such tables given by Benjamin Ball (b. 1833) and Chambard in their classical article " Sjomnambulisme " in the Dictionnaire encyclopedique des sciences medicales, shows the relation of the various intermediate conditions of sleeping and awaking and of the dreaming and somnambulistic states. The horizontal stroke indicates the presence of the condition the name of which heads the column:—
| Organic | Conscious- | Imagin- | Co-ordi- | Power of movement and sensibility. | |
| Normal waking state | —— | —— | —— | —— | —— |
| Sleep, 1st degree.. . ″ 2nd degree . ″ 3rd degree. . | —— —— —— | —— —— —— | —— —— | —— | |
| Deep sleep. . | —— | ||||
| Waking, 1st degree ″ 2nd degree (specially ″ 3rd degree. . | —— | —— | | | |
| Complete waking . Dreaming state. . | —— | —— | —— | ||
| Ordinary somnambulism | —— | —— | —— | —— | |
| Profound somnambulism (perfect unconsciousness) . | —— | —— | |||
| Somnambulistic dream (movements in a dream) | —— | —— | —— |
The following table, modified from two such tables given by Benjamin Ball (b. 1833) and Chambard in their classical article "Somnambulisme" in the Dictionnaire encyclopedique des sciences medicales, shows the relation of the various intermediate conditions of sleeping and awaking and of the dreaming and somnambulistic states. The horizontal stroke indicates the presence of the condition the name of which heads the column: The somnambulist acts his dream. His condition is that of a vivid dream in which the cerebrum is so active as to influence centres usually concerned in voluntary movements. Under the dominant idea he executes the movements that this idea would naturally excite in the waking state. Many of his movements are in a sense purposive; his eyes may be shut so that the movements are executed in the dark, or the eyes may be open so that there is a picture on the retina that may awaken no consciousness, and yet may, by reflex mechanisms, be the starting-point of definite and deliberate movements. In many cases he does not hear, the auditory centres not responding; but in others suggestive words may alter the current of his dream and lead him to perform other actions than what he intended to do. On awaking there is either no memory of what has taken place or the dim recollection of a fading dream.
It is important to notice that there is scarcely any action of which a somnambulist may not be capable, and immoral acts from which the individual would shrink in waking hours may be performed with indifference. Considering the abrogation of self-control peculiar to the physiological condition, it is evident that no moral responsibility can be attached to such actions. In cases where somnambulistic propensities place a person in danger, an endeavour should be made to induce him to return to bed without awaking him; as a rude awakening may produce a serious shock to the nervous system. Inquiry should then be made into the exciting cause of the somnambulistic dream, such as a particular train of thought, over-excitement, the reading of special books, the recollection of an accident or of a crisis in the person’s history, with the view of removing the cause if possible. It should never be forgotten that somnambulism, like chorea, hysteria and epilepsy, is the expression of a general morbid predisposition, an indication of a nervous diathesis, requiring careful treatment so as to avoid more dangerous maladies.
See also Sleep and Muscle and Nerve (physiology).