Here it is essential to the safety of the individual that he shall respond in sleep, not merely to sounds or movements which threaten danger, but also to the sounds or movements of the other members of the group. Moreover, it is necessary that this response shall be discriminative and selective. If each member of the group awakened in response to any kind of stimulus, it would conflict seriously with the recuperation which is the special function of sleep. The power of discrimination and selection which is shown in the process of awakening in Man may be regarded as the direct descendant of the similar power which is essential to the safety of gregarious animals. A moment's consideration will show that the principle does not apply.
Sleep is a definitely, and even finely, graded process.
It is often difficult to say whether a person, including oneself, is or is not, or has or has not been, asleep. Moreover, the process of waking, as I have just said, implies the presence of the processes of discrimination and selection. I have regarded sleep as allied to the instinct of immobility, but we can now see the essential difference between the two. If I am right, sleep is an example of the instinctive process of suppression in which this process has become capable to a very high degree of being graded and of reacting to delicately-discriminated and selected stimuli. Since this power of gradation and selection appear especially in relation to awaking, and since the process of awaking stands in a close relation to the welfare of the group, it is no great assumption to suppose that the grading stands in a definite relation to gregarious needs and to the process of suggestion by means of which these needs are satisfied.