The next class of Vrittis is called sleep and dream. When we awake, we know that we have been sleeping; we can only have memory of perception. That which we do not perceive we never can have any memory of. Every reaction is a wave in the lake. Now, if, during sleep, the mind had no waves, it would have no perceptions, positive or negative, and, therefore, we would not remember them. The very reason of our remembering sleep is that during sleep there was a certain class of waves in the mind. Memory is another class of Vrittis which is called Smriti.