How you pay for work performed is a complex matter. It's one that Adam Smith covers (as he does most topics) at length in Wealth of Nations. In particular he warns of the tendency to pitch compensation terms such that workers are overincentivized to work, reducing their overall productivity and burning out faster.

http://www.gutenberg.org/files/3300/3300-h/3300-h.htm

(I'm adding some paragraph breaks to Smith's text, this is a single paragraph in the original. Emphasis added as well.)

"The liberal reward of labour, as it encourages the propagation, so it increases the industry of the common people. The wages of labour are the encouragement of industry, which, like every other human quality, improves in proportion to the encouragement it receives. A plentiful subsistence increases the bodily strength of the labourer, and the comfortable hope of bettering his condition, and of ending his days, perhaps, in ease and plenty, animates him to exert that strength to the utmost. Where wages are high, accordingly, we shall always find the workmen more active, diligent, and expeditious, than where they are low; in England, for example, than in Scotland; in the neighbourhood of great towns, than in remote country places.

"Some workmen, indeed, when they can earn in four days what will maintain them through the week, will be idle the other three. This, however, is by no means the case with the greater part. Workmen, on the contrary, when they are liberally paid by the piece, are very apt to overwork themselves, and to ruin their health and constitution in a few years. A carpenter in London, and in some other places, is not supposed to last in his utmost vigour above eight years. Something of the same kind happens in many other trades, in which the workmen are paid by the piece; as they generally are in manufactures, and even in country labour, wherever wages are higher than ordinary. Almost every class of artificers is subject to some peculiar infirmity occasioned by excessive application to their peculiar species of work. Ramuzzini, an eminent Italian physician, has written a particular book concerning such diseases.

"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."