Does working more hours always equal more work being completed?

Adam Smith, father of modern economics, says no:
"We do not reckon our soldiers the most industrious set of people among us; yet when soldiers have been employed in some particular sorts of work, and liberally paid by the piece, their officers have frequently been obliged to stipulate with the undertaker, that they should not be allowed to earn above a certain sum every day, according to the rate at which they were paid. Till this stipulation was made, mutual emulation, and the desire of greater gain, frequently prompted them to overwork themselves, and to hurt their health by excessive labour. Excessive application, during four days of the week, is frequently the real cause of the idleness of the other three, so much and so loudly complained of. Great labour, either of mind or body, continued for several days together is, in most men, naturally followed by a great desire of relaxation, which, if not restrained by force, or by some strong necessity, is almost irresistible. It is the call of nature, which requires to be relieved by some indulgence, sometimes of ease only, but sometimes too of dissipation and diversion. If it is not complied with, the consequences are often dangerous and sometimes fatal, and such as almost always, sooner or later, bring on the peculiar infirmity of the trade. If masters would always listen to the dictates of reason and humanity, they have frequently occasion rather to moderate, than to animate the application of many of their workmen. It will be found, I believe, in every sort of trade, that the man who works so moderately, as to be able to work constantly, not only preserves his health the longest, but, in the course of the year, executes the greatest quantity of work.

In cheap years it is pretended, workmen are generally more idle, and in dear times more industrious than ordinary. A plentiful subsistence, therefore, it has been concluded, relaxes, and a scanty one quickens their industry. That a little more plenty than ordinary may render some workmen idle, cannot be well doubted; but that it should have this effect upon the greater part, or that men in general should work better when they are ill fed, than when they are well fed, when they are disheartened than when they are in good spirits, when they are frequently sick than when they are generally in good health, seems not very probable. Years of dearth, it is to be observed, are generally among the common people years of sickness and mortality, which cannot fail to diminish the produce of their industry."

From Wealth of Nations, p 57 of 641 (in my edition)