Language

Consider this description of Collins attempting to fall asleep:

He lay and the darkness came up to him, the darkness spread out to the edges of his being, the darkness washed away the edges of his being as the sea melts the edges of the sand.  But just as it was about to smooth out his head and wash down the pinnacles of his features like a sandcastle, a return of consciousness rose within him and swept it away.  (p. 159)

To what is falling asleep compared?  What does the comparison suggest to us about falling asleep?  do you find this extended metaphor appropriate?  If not, what comparison would you make?