As if you could kill time without injuring eternity.

Hours are not a substance you have spare. Wishing an afternoon away, or filling it with something you will not remember, spends part of a total that is fixed and unknown to you, which is what makes the casual phrase about killing time slightly grim once you look at it directly.

None of which argues against rest, and doing nothing on purpose is not the same as wasting anything. The distinction is whether you chose it. Notice the hours you are trying to get through rather than spend, and see whether one of them could become something you would rather have.