To what extent can this generalize from flies to humans? I've been very interested in dreams and read a decent amount of research on sleep and its functions, but most of that was years ago so my knowledge may be outdated. But my impression was that there are non-negligible differences in how sleep works (e.g., in terms of brain activity) between say, birds and mammals, or even one mammal to another. Certainly there could be some basal functions that are shared in flies but it seems a stretch to say "it all comes down to" that. As someone said in another comment, it's unclear what makes this about sleep rather than rest.