In the last chapter I have made no reference to an aspect of hypnotism which is of much importance and must now be considered. Not only is the suggestion to sleep one of the most frequent measures which are used when it is desired to produce the hypnotic state, but when this state has been produced, it is often difficult to distinguish it from normal sleep, the chief difference being that the time-limit of the sleep may be determined by the suggestion of the hypnotist and has not the free character possessed by the customary process of sleep. We may assume with some confidence that sleep is a state which is allied to hypnotism, and therefore stands in some relation to the process of suggestion. This raises the problem of the nature of sleep, which I must now consider.
Sleep is an aspect of life which has been strangely neglected by the psychologist.
Students of psychology are profoundly interested in the dreams which occur during sleep and in their more abnormal varieties, such as nightmares and somnambulism, but they seem to have been content to regard sleep itself as a process which belongs to the realm of physiology. They fail to recognise that sleep is much more than a mere negation of psychological activity, and that, quite apart from the occurrence of dreams, sleep has characters of a positive kind which must be fitted into any scheme of mental happenings which seeks to be consistent and complete. I have already mentioned the resemblance of the hypnotic state to sleep. I have now to consider other points in which sleep resembles the state which follows the hypnotic suggestion of another person. Several features of sleep have led students to believe that it is the result of suggestion exerted by the conditions of time and space with which sleep is habitually associated.