The knowledge 'I am' is the very seed of memory and all information functions through it, it forms the basis of the mind. It is bound to tire out and hence there is sleep, otherwise you would die if you didn't sleep. But sleep is not the complete departure of the 'I am' it's only held in abeyance and after sleep it gets refreshed and starts its activity again maintaining the continuity. No wonder if your name is called aloud in sleep you wake up and respond saying 'that's me'! Physical death is the total departure of the 'I am' and nothing is retained. For the 'Realized One' who has transcended the 'I am', memory and the 'I am' are available to him, he may or may not use them, they are not 'lodged' in him anymore. Only the 'Realized One' can understand this state.