Not necessarily. A reduction in working time can benefit employees and employers if it avoids working when the required effort is high. The key point is not the average effort over the week, but the fact that it can increase sharply with the number of days spent on the task.
A five-day week with two sectors
To illustrate the point, let us consider an economy with two sectors, offices and restaurants (the debate on the four-day workweek is focused on the latter). The figures below show the daily wage (blue lines) and the daily effort cost (red line) according to the number of days worked.
In offices (above), the cost is constant during the week: working on Friday is not really more difficult than working on Monday. The red line is thus horizontal. We consider a daily wage of 200 and an effort cost equivalent to 80 in monetary terms, so that the employee gets a net well-being of 120 per day. Working a five-day week in an office then gives a net well-being of 600 per week, which is the area between the blue line and the red line.
The figure above shows the situation in the restaurants. The key point is that the effort cost increases sharply on the fifth day, as the fatigue of the specific working hours of this job is felt, bringing with it its share of inconvenience and even the risk of accidents. More precisely, the cost is 80 for each of the first four days, then rises to 260 for the fifth, i.e. an increase of 180. The salary is 236 per day. This figure is not random, but is the amount needed to make the net well-being of a five-day week the same in both sectors (i.e., 600). If this were not the case, people would change sectors. The cost to the employer in the restaurant industry is the total wage paid, i.e. 1,180 (five days at 236)
Moving to a four days workweek
What happens if the work week in a restaurant is shortened to four days? First, we need to calculate the new wage, and then the weekly cost to the employer of hiring another person for the fifth day.
As before, the working conditions in the restaurant must be such that the staff gets the same net well-being as they would in an office job, i.e. 600 per week. This implies a daily wage of 230, which is a decrease from the five-day week. The weekly income drops even more sharply, from 1180 (five days at 236) to 920 (four days at 230). Why would staff accept this drop in income? Simply because stopping after four days allows them to avoid the heavy effort of the fifth day, which in the end compensates for the lower income.
The employer also wins. To ensure the weekly service, he must have staff on each of the five days. The weekly cost is reduced to 1'150, thanks to the lower salary. Therefore everyone wins. If the reduction of the working time allows the employees to be more productive on the four days, the employer gains even more.
One needs to think in terms of marginal effort cost
A more general analysis than this example identifies the condition that must be met for the four-day week to be beneficial. It requires that the difference between office wages (WageOffice, 200 in our example) and office effort cost (EffOffice, 80 in our case), adjusted by the different number of days worked in the two sectors, is smaller than the extra cost of the fifth day in the restaurant industry (ExtraRest, 180 in our illustration):
(5/4)×(WageOffice - EffOffice) < ExtraRest
This shows that the relevant notion of hardship in the restaurant industry is not the average effort cost during the first four days, which does not appear in the equation, but the extra cost on the last day, which economists call marginal cost. Reducing the week makes it possible to avoid situations of high effort cost, which is ultimately an efficient choice.
The example considered is of course very simplified. Nevertheless, it allows us to identify the right question to ask in the discussion of moving from a five-day week to a four-day week: does the daily effort cost of work in the sector increase sharply beyond a certain threshold of days worked?
>> Detailed analysis in appendix <<
