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Often in the rhetorical question how can (also do) you sleep at night?, the phrase to be able to sleep at night, and its variants, mean: to be untroubled by the moral consequences of one’s actions, or by the risks and responsibilities of one’s situation.

This phrase occurs, for example, in an open letter to George Walker Bush (born 1946), President of the USA from 2001 to 2009, by one Ron Setzer, published in the Missoulian (Missoula, Montana, USA) of Thursday 20^th November 2003 [page A7, column 1]:
—Note: The Harken Energy scandal involved allegations of insider trading by George Walker Bush:

Some patriot you are. While I’m at it, some Christian you are. I don’t know how you sleep at night. You hid from service in Vietnam and now you send Americans to die in order to increase the wealth of you and your friends; you got into schools because of special privilege instead of merit; you did drugs and lied about it; you committed what you now call corporate crime even as you sold out stockholders in Harkin [sic] Oil—a company you helped run into the ground; you and your cronies are rolling back decades of environmental regulation progress for the short term financial gains of a few; you act like you have a mandate to do whatever you want as president, even though you didn’t even win the popular vote in a corrupt election. And to top it all off, you lie about everything. Like I said, I don’t know how you sleep at night.

These are, in chronological order, the earliest occurrences of the phrase to be able to sleep at night and variants that I have found:

1-: From The Son of Commerce, an original poem, in thirty-four cantos, written by a sailor (London: Moore & Son, 1806) [Canto XVIII: Son of Commerce turned doctor; page 64]:

Now doctors, sirs, like you, live by their trade,
They don’t pay visits, but visit to be paid.
Yet e’er I do begin my arduous task,
Some necessary questions I must ask;—
First tell me, masters, how you sleep at night?
Does Incubus your conscience mount to fright?
Or Morpheus on the wall of fancy paint
Such distant objects, as your minds attaint,
To break that sweet repose (indulgent heav’n,)
In kind compassion unto man has given?

2-: From the Western New-Yorker (Warsaw, New York, USA) of Tuesday 1^st November 1853 [page 1, column 4]:
—Note: This text was reprinted in Country Margins and Rambles of a Journalist (New York: J. C. Derby, 1855), by the U.S. authors Samuel H. Hammond (1809-1878) and Lewis William Mansfield (1816-1898):

D, WITH A DASH TO IT.

Oh, compositors! oh, proof-readers, and you, oh, printer’s devils! […].
[…]
[…] When you merely print on for or, as in a late paper of ours, a shrewd reader may discover the error; but when you drop whole words, containing the heart of a paragraph, I can scarcely express my astonishment. How you can sleep at night, after putting forth such fragments to the world, it is difficult to say.

3-: From a review of Bibliotheca Classica. Demosthenes, with an English Commentary. By the Rev. Robert Whiston (London: Whittaker and Co., and George Bell, 1859)—review published in The Saturday Review of Politics, Literature, Science, and Art (London, England) of Saturday 3^rd September 1859 [page 285, column 2]:
—In the following, the reviewer comments on Robert Whiston’s “galimatias”:

We are less concerned with the versions quoted by the learned editor, than with those which are the fruits of his own ingenuity. […]
[…]
[…] We will give our readers the benefit of another sentence which might have issued from the Whistonian school of Rhetoric:—“I say, Tom,” the guard of a mail-coach was once heard to say to the horse-keeper, “I say, Tom, I can’t tell how you sleeps o’ nights, owing such a sight o’ money as you does!” “Well, Jack,” was the answer, “if them sleeps as I sleeps as I owes money to, I can sleep all right!” Si non è vero, è non mal trovato; and if the reply is not a specimen of Mr. Whiston’s English, it ought to be.

4-: From the transcript of a speech delivered by Joseph A. Wright (1810-1867), United States Senator from Indiana, published in the Daily State Sentinel (Indianapolis, Indiana, USA) of Saturday 27^th December 1862 [page 2, column 1]:
—Note: The Knights of the Golden Circle, a secretive organisation created in 1854, proposed to establish a slaveholding empire encompassing the southern United States, the West Indies, Mexico and parts of Central America:

“These traitors of the Democratic party pretend to be the disciples of Jackson—pretend to advocate the doctrines of Benton and myself! Only think of it! They are nearly all of them traitors, and if you elect a majority of them to your Legislature, they will carry Indiana over to the Southern Confederacy! They mean to do it—they will do it, for nearly every one of them are Knights of the Golden Circle. My old Democratic friend there—you venerable man with the grey hair and venerable locks—how can you sleep at night, with such a prospect before you?”