Why is this premise of this 40 hours compressed into 4 days? To me, reduced hours means making more of the time you actually spend working. 40-50 work weeks in

Why is this premise of this 40 hours compressed into 4 days? To me, reduced hours means making more of the time you actually spend working. 40-50 work weeks involve a lot of thumb twiddling and doing nothing in between the impactful stuff.