“When we consider the immense human significance of sleep, the absolute necessity for us to spend a considerable part of our lives in abject mental annihilation, it is remarkable how little we know about it. …” In these words, Sir John Eccles introduced the recently published Ciba Foundation Symposium on “The Nature of Sleep” (3), in which are described many of the advances which have been made in recent years in our attempts to understand the phenomena which are involved in the state of sleep. In one of the papers included in this Symposium, Bremer (2) reviews the work which has been reported since 1954 on the neurophysiological mechanisms involved in sleep. The relationship of dreaming to the depth of sleep has been investigated by such workers as Kleitman (5), Dement (4) and Wolpert (11). These studies have been made possible by the application of modern techniques of measuring mental activity. The use of electroencephalographic methods has allowed an approximate classification of the different levels of sleep which enables different workers to adopt a standard frame of reference. Measurements of rapid eye movements and of changes in muscle potential during sleep have provided more objective methods of assessing dream activity. Other recent studies have attempted to make an objective assessment of the physical and mental effects of sleep deprivation (Morris et al., 7, Bliss et al., 1, Murray et al., 8, Wilkinson, 9, Williams et al., 10).