Choose the word that represents the antonym of "heaviness":

“There is a kind of sleep that steals upon us sometimes, which, while it holds the body prisoner, does not free the mind from a sense of things about it, and enable it to ramble at its pleasure. So far as an overpowering heaviness, a prostration of strength, and an utter inability to control our thoughts or power of motion, can be called sleep, this is it; and yet we have a consciousness of all that is going on about us; and if we dream at such a time, words which are really spoken, or sounds which really exist at the moment, accommodate themselves with surprising readiness to our visions, until reality and imagination become so strangely blended that it is afterwards almost a matter of impossibility to separate the two. Nor is this, the most striking phenomenon, incidental to such a state. It is an undoubted fact, that although our senses of touch and sight be for the time dead, yet our sleeping thoughts, and the visionary scenes that pass before us, will be influenced, and materially influenced, by the mere silent presence of some external object: which may not have been near us when we closed our eyes: and of whose vicinity we have had no waking consciousness. ” 

— Charles Dickens, Oliver Twist