"the centrality, the spontaneous energy, of the ego becomes enervated in dreams"

   The translation of 'ich' as 'ego' here could be seen as premature - while something like Freud's famous three-part system of the mind is certainly alluded to later in the book, he would not give it its familiar terms until his 1920 essay Beyond the Pleasure Principle.  It was in the English edition of this that translator James Strachey first applied the Latinised 'id', 'ego', and 'super-ego' - Freud himself simply used the German words for, respectively, 'the it', 'the I', and 'the over-I'.

Online edition of Sigmund Freud's 'Beyond the Pleasure Principle', translated by James Strachey, (1920)