It really depends on the situation. On the one hand, you surely have more precious time to work on side-projects or freelancing (if you are employed in a company), or just invest in self-improvement, learning, or even an online degree (like Udacity or Coursera).
But on the other hand, some companies may force all their employees to have 4-day workweeks which will result in a significantly smaller house income, even for families with loads, children, etc...
As a relatively young employee with a passion for online learning I say yay, but as a father (not yet) with many financial responsibilites, I can easily imagine myself saying nay.
As a father myself, I've reached the opposite conclusion. I would readily trade off 20% of my income (and reduce my monthly nut accordingly) with an additional day with my kiddos (or with myself, come to that). 😄
My wife and I have done just this. Both reduced to 4 days a week to spend a day (without the other person) a week spending quality time with our little-ish one.
I'd like to say we just bit the bullet and took the 20% reduction, but for us it actually worked out financially better to have the 20% less per week than pay for someone, or somewhere, for our little one to go whilst we worked.
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