Sleep; when the mind withdraws from the external world, it enters into deep sleep of Sushupti, on account of the overpowering influence of Samsara. The Sadhaka should arrest this tendency and attempt to fix the mind on to Atma-vichara, or the inquiry into the nature of the Atman. He must keep watch over the mind, so that he may keep aware. (JV, p. 4)