The hypnoidal state may be said to partake not only of the nature of waking and sleep, but also to possess some characteristics of hypnosis, namely, _suggestibility_. It is clear that, from the very nature of its mixed symptomatology, the hypnoidal state is variable and highly unstable. The hypnoidal state may be regarded in the light of an equivalent of sleep. Like sleep, the hypnoidal state has many levels of depth. It differs, however, from sleep in the rapidity of oscillation from level to level. In the experiments of various investigators, the depth of sleep is found to be represented by a rapidly rising curve during the first couple of hours, and by a gradually descending curve during the rest of the hours of sleep. No such regularity of curve can be found in the hypnoidal state. The depth of the hypnoidal state changes very rapidly, and with it the passive condition and suggestibility of the patient. For many years investigations of the hypnoidal state were carried out by me on subjects and patients, adults, and children. The work was entirely limited to the study of such states as found in man. Having found that during the hypnoidal state the condition of suggestibility is quite pronounced for therapeutic purposes, and having effected many cures of severe psychopathic maladies ranging throughout the whole domain of hysterical affections, neurasthenia, obsessions, drug habits, especially alcoholic ones, the hypnoidal state has become, in my practice, quite an important therapeutic agent. Other investigators have obtained some excellent results with the hypnoidal state in their treatment of various functional, psychopathic maladies. Thus far, the work with the hypnoidal state has been confined entirely to observations and experiments on human subjects and patients, and also to the treatment of man’s psychopathic ailments. I undertook a series of experiments on sleep, both from a phylogenetic and ontogenetic standpoint, following up the conditions and manifestations of sleep in the ascending scale of animal life, from the frog and the guinea pig, through the cat, the dog, to the infant and the adult. My experiments clearly prove that the hypnoidal state is by no means confined to man, but is also present in animals. This is important since it indubitably shows how widely spread the hypnoidal state is throughout the domain of animal life. Moreover, the experiments clearly prove that the further down we descend in the scale of animal organization, the more prominent, the more essential, does the hypnoidal state become.