[rtpg]

"You're cutting productive and unproductive hours at the same time" is perhaps true, but it's about the ratio. Obviously at the extremes (cutting the last day of work vs going from a 7-day to a 6-day) there's going to be differences.

Acting like unproductive hours are some random result goes against loads of companies who have been doing actual trials of 5 => 4 day weeks. Having time off is actually helpful, and the sort of procrastination from dissatisfaction/exhaustion/whatever that leads to this can be helped by this kind of stuff!

"Why is it 4 and not 3 or 2 or whatever" I mean it probably depends on people but TGIF is a saying for a reason

[breuleux]

I don't think unproductive hours are random, but I suspect that for mental work the productivity ratio decreases more dramatically in the last hour of each day than the last day of a week. Basically, from a pure productivity standpoint, it may be more efficient to go from 5x8 to 5x6 (or even 5x4) than to go from 5x8 to 4x8. Most people might prefer 4x8 because a full day off is more flexible, however.

[ProZsolt]

You need to calculate the productivity boost on Mondays after an extra day of rest.