Letters. He included it in a letter he wrote to his great friend, Thomas Moore, from Venice on February 28th 1817. Byron tells Moore about Carnival season in Venice, "At present, I am on the invalid regime myself. The Carnival - that is, the latter part of it, and sitting up late o'nights, had knocked me up a little. But it is over - and it is now Lent, with all its abstinence and sacred music." He feels tired & worn out, at the age of twenty-nine.
If I live ten years longer, you will see, however, that it is not over with me - I don't mean in literature, for that is nothing; and it may seem odd enough to say, I do not think it my vocation. But you will see that I will do something or other - the times and fortune permitting - that, 'like the cosmogony, or creation of the world, will puzzle the philosophers of all ages'. But I doubt whether my constitution will hold out. I have, at intervals, exorcised it most devilishly.
The rest of the letter is about literary matters, gossip about reviews in English periodicals & enquiries about Moore's own plans for publication.
So, we'll go no more a-roving
So late into the night,
Though the heart be still as loving,
And the moon be still as bright.
For the sword outwears its sheath,
And the soul wears out the breast,
And the heart must pause to breathe,
And love itself have rest.
Though the night was made for loving,
And the day returns too soon,
Yet we'll go no more a-roving
By the light of the moon.