Although falling asleep is listed as one of the obstacles to yoga by Patanjali, whether it really is so depends both on the kind of sleep and the circumstances in which it develops. If very deep and very refreshing, it has some positive value--either in conferring temporary peace of mind or in healing some bodily ill. And if it occurs while practising conjointly with and in the presence of a master, it is definitely conducive to spiritual progress. But any other kind is certainly a waste of meditational time. To prevent its happening, or to arouse the sleeper from it if it has already happened, the Japanese Zen monks sitting in the meditation hall are supervised by a prefect who either slaps the drowsing man on the shoulder with the broad end of an oar-like pole or else rings a bell every twenty minutes. A different method is used in Siam and Ceylon by monks who meditate in solitude. A few pieces of wood are fastened to a candle about one inch apart. As the candle burns down, the pieces fall at intervals, thus awakening the monk if he is asleep.
-- Notebooks Category 4: Elementary Meditation > Chapter 1 : Preparatory > # 486
